Here's my opinion;

Top 5 Best

5. 2000-2004
Firstly, this time period was Eminem's zeitgeist, as he would release "The Marshall Mathers LP", "The Eminem Show", as well as a soundtrack from "8 Mile", a movie he starred in.
Second, this was a transitional period between Y2K era nu metal and core 00s alternative rock/metal. Bands of this nature include Linkin Park, Papa Roach, System of a Down, Breaking Benjamin, Evanescence, and Three Days Grace. Hell, even Nickelback did have a great metal song in this period, "Because of You".  At the end of this time period came Green Day with the smash hit "American Idiot".
Third, this time period musically holds a lot of nostalgia for me, as it's the first time period I remember. The soundtrack of SSX3 is a perfect example of this.

4. 1970-1974
At the start of this time period in 1970, Black Sabbath released their self titled album, often considered to be the first metal album. A year later in 1971, you had Led Zeppelin's "Led Zeppelin IV" and The Who's "Who's Next", two critically acclaimed albums that contained the smash hits "Stairway to Heaven" and "Baba O'Riley" respectively. Following year, 1972, you had Deep Purple release "Machine Head", the same album with the blues metal masterpiece "Smoke on the Water", the same song with the riff that practically every electric guitar player tries to replicate. Then in 1973, you had Pink Floyd release "the Dark Side of the Moon", which went on to be one of the best selling albums of all time, as well as the debut of Aerosmith and Lynyrd Skynyrd, each with their own self titled albums featuring "Dream On" and "Free Bird" respectively. This truly was one of the most influential time periods in history for music.

3. 1975-1979
At the start of this time period in 1975, you had Queen become successful with the masterpiece "Bohemian Rhapsody", as through the rest of the period they would go on to release other great songs such as "We Are the Champions" and "Don't Stop Me Now". In 1976, you had The Eagles "Hotel California", as well as The Ramones kicking off punk rock with their self titled debut album, this punk rock movement would continue throughout the rest of this time period with groups such as The Sex Pistols and The Clash. Then, in 1977, you had Judas Priest revolutionize metal with the album "Sin After Sin", and a year later in 1978 you had Van Halen release their debut album.

2. 1980-1984
This time period right here was the zeitgeist of the New Wave of British Heavy Metal. In 1980, both Iron Maiden and Def Leppard released debut albums, Judas Priest released "British Steel" (the same album with "Living After Midnight"), Motorhead released "Ace of Spades", and Saxon released "Wheels of Steel". All of that in only one year. More great albums would come out from these bands in the later years of this time period, the best ones of course being Iron Maiden's "Number of the Beast" and "Piece of Mind".
Not only was NWBHM great in this time period though. Thrash metal also made its debut in this period, with two debut albums; Metallica's "Kill 'Em All", and Slayer's "Show No Mercy" in 1983. Following year in 1984 you had the debut of Anthrax, as well as Metallica release their "Ride The Lightning" album.
Outside of rock and metal, this was also a great time for reggae. UB40 released the album "Labour of Love" in 1983, which contained the mass hit "Red Red Wine", and in 1984 you had "Legend", the compilation album of songs from Bob Marley & The Wailers.

1. 1990-1994
Easily the best.
For one, this is when grunge broke into the mainstream with successful albums such Nirvana's "Nevermind" and Pearl Jam's "Ten", as well as masterpiece albums from Soundgarden and Alice in Chains. You also had "Blood Sugar Sex Magic" from the Red Hot Chili Peppers, Metallica's self titled black album, as well as great groups such as Nine Inch Nails and the Cranberries in this time period. This was the time period in which rock music became far more raw and edgy, which is definitely a great thing.
Outside of rock, you also had the golden age of rap. 2Pac released his debut album in 1991, and G-Funk started out in 1992 when Dr. Dre released "The Chronic". Later in this time period, you had the emergence of east coast hip hop with Wu-Tang Clan's "Enter the Wu-Tang (36 Chambers)", and Notorious BIG's "Ready to Die".


Top 5 Worst

5. 1985-1989
Mostly because this was the time period in which rock and metal became far too commercialized. This is exactly what the grunge movement was in retaliation against, rock and metal became far too mainstream and far too conforming in this time period.  The main offender here is glam metal, or hair metal, whichever you want to call it. The popularity of cheesy cliche hit rock songs such as Guns N Roses "Welcome to the Jungle", Motley Crue's "Girls Girls Girls", and Bon Jovi's "I'll Be There For You" are a reflection of this.

4. 2005-2009
The reason this one is higher than #3 is because the alternative rock/metal scene in this period I thought was still good, and it was still mainstream.
What gets this period on this list is definitely the over abundance of godawful ringtone hip hop, and later on the arrival of garbage acts such as Katy Perry, Lady Gaga, and 3OH!3.

3. 1995-1999
At the start of this time period you had the peak of britpop, a genre I've never really liked, as I personally think too many britpop bands tried too hard to replicate The Beatles. However, what really gets it on this list is the arrival of cheesy cringe-worthy Y2K teen pop, which started out with the Spice Girls in 1997 when they released "Wannabe", then with garbage from the likes of The Backstreet Boys and Britney Spears. Also, rock music heavily deteriorated in this time period, with the arrival of soft post-grunge puss-rock from the likes of Matchbox Twenty and Goo Goo Dolls. Plus the nu metal scene from this time period also sucks, Limp Bizkit is way too douchy, and my problem with Korn is that Jonathan Davis's lyrics are just cringe-worthy awful, hell they even have a song called "Ass Itch", like wtf?!
Some albums I do like from this time period though are the ones that came out from 2Pac, Notorious BIG, Foo Fighters, and The Offspring, they're all good.

2. 2010-2014
The 10s in general is just garbage awful for mainstream radio-play music. It's a great decade for underground metal because now bands have to be good to appeal to a certain fanbase since they can no longer rely on radio promotion, but as far as mainstream music goes, it's just awful. Electropop was just cringe-worthy, and I also wasn't too fond of the indie movement that came along later in this time period.

1. 2015-2019
Even worse.

What are your top 5 favorite and least favorite half decades for music?

Every period has its fair share of greatness, so I gonna limit myself to the mainstream.

Best:
1. 65-69: Peak of Psychedelia, as well as ultra cheesy 60s Bubblegum :D
2. 80-84: New Wave and Synthpop are taking over, and what a great inventive time it is!
3. 00-04: Early 2000s R&B ftw.
4. 95-99: A really fun period filled with cheesy Europop, Vocal Trance and Teen Pop.
5. 75-79: I mean Disco, and Prog Rock, and Disco, and Post-Punk, and Disco.

Worst:
5. 85-89: Power ballads that sound the same as well as the extremely wannabe-tough but still very poppy Glam Metal, yuck.
4. 60-64: The boring kind of 60s bubblegum, still plenty of crooning stuff and all the Beatles-wannabees trying to grab the same succes and failing miserably.
3. 05-09: Awkward transitional period containing the infamous Ringtone Era, still plenty of bops though.
2. 90-94: This era of mainstream music just mesmerizes me by its awfulness. Lots of second-rate Pop Rap songs, faux-emotional and forced adult contemporary ballads, the last remains of 80s Glam Metal and supra annoying Dance-Pop anthems. I just can't
1. 50-54: No, no, I really can't. All the slow and plotting Vocal Jazz songs, the crooning Traditional Pop ballads, the cliched trope of "warm voices", it's way too boring for me. Even Jazz was in a transitional phase that wasn't really anything.

Best:

1. 1995–1999: A consistently phenomenal period on pretty much all fronts. Truly a glorious time for music that I wish I could personally experience at an older age than young child.

2. 1965-1969: This ranks highly because like #1, it’s remarkably consistent in quality, even though it doesn’t contain quite as much variety or as many full-on classics to me personally.

3. 2000-2004: This period is a lot better than I originally recalled. There was some serious quality music with bucketloads of variety, and unlike the second half of the 2000s, there wasn’t enough crap to set its overall consistency back too far.

4. 1980-1984: The first quarter of this decade was mostly a bummer for the US, but everything else was truly fantastic.

5. 1985-1989: There was an aura of shallowness within the vicinity of 1987, but the ‘80s were still the ‘80s.

Worst:

1. 2015-present: The infestation of apathetic trap and formulaic EDM make this a truly forgettable age of music. I hate it. I sure wish I lived in an era in which music was still a big deal as opposed to everyone but me being network/streamed television junkies.

2. 1960-1964: This is spared the #1 dishonour because 1964 is a great year for music, but the rest is dull and formulaic. Much like the 2010s, the early 1960 were a far more relevant time for television and cinema than music.

3. 1955-1959: Sorry, I’m just not a fan of ‘50s music. The few biggest classics of early rock & roll are not enough to save an overall repetitive, outdated, and corny time for music.

4. 1945-1949: I don’t generally like judging music eras from before the advent of rock & roll because you have to understand the context, but even during the 1940s, music in general took a serious nosedive in quality and ambition compared to the innovative, melodious, and passionate material to emerge out of the Harlem Renaissance of the 1920s and 1930s. During the G.I. Era, the music industry got incredibly complacent and banked primarily on the stodgiest, most inoffensive manufactured music, too afraid to let in any creativity due to the McCarthyite social climate. The first few years after World War II were the very worst because not only was music forgettably tasteless, but sound quality was notably worse than it was during the otherwise virtually identifcal first half of the 1950s, so the post-War decade’s one major asset of producing largely soothing music was far more realized during those years, whereas the mid-to-late 1940s are virtually useless by later standards. Hank Williams may be the only true saving grace for this half-decade.

5. 2005-2009: The first few years of this period contain a lot of really excellent music, but unfortunately, all the surrounding mediocrity and utter garbage puts it at an ultimate disadvantage.

2005 to 2009 was the worst era of music.....ever!

This is difficult for me because I don't really have a specific half-decade that I like and dislike. There are certain years that I liked and disliked for music. For example, I liked 2010-2013 but I didn't like 2014, but I liked 2015-2017.

Top 5 (since 1950)

1) 1965-1969 (Just go to your music store's bookshelf, the artists who came of age then are the most popular by far, and even if you're old when you were young then, you'll still like the electricity that has never been matched since).
2) 1970-1974 (Progressive rock, Philly Soul music, singer-songwriters, jazz fusion, and even the novelty songs were fun. The Baby Boomer era had the best music).
3) 1980-1984 (Got to love the fun New Wave/early synthpop, electrofunk, and even old school rap and the romantic ballads of the era)
4) 1975-1979 (Punk, funk, early New Wave fun disco, arena rock, early Springsteen, with some singer-songwriters still around)
5) 1955-1959 (Modern "cool" jazz was at its height, and it was more than just early rock and there were some great musicals like West Side Story and instrumentals like Peter Gunn and Cherry Pink and Apple Blossom White)

Bottom 5

1) 2015-2019 (projected, don't fall into the crap that's Trap, and I don't even watch the awards shows anymore, not to mention that what was once known as "oldies" on the radio is starting to play more late '80s and '90s and no more '60s and less '70s)
2) 2010-2014 (Dubstep? EWWW)
3) 2005-2009 (More crunk with snap rap peaking, and autotune taking a big spike in usage later onwards)
4) 1995-1999 (1995-96 were OK, but becomes TRASH when that first Hanson record drops, leading to the most "fake pretentious" period in pop_
5) 2000-2004 (Good underground scene and some OK mainstream groups like Coldplay along with the waning Nu Metal scene, but not a fan of glam rap)

All those Millennial G-E-G "wo oh wo"s, ProTools, increased compression, and executives pushing for the similar sound have honestly ruined music. Sure, there are some hidden gems like in the early 2000s during Indie Rock's heyday, but you can't deny the period from Elvis up through the early years of MTV. The first half of the '60s and second half of the '80s are good "guilty pleasure" eras while the early-mid '90s has some good stuff, but still pales to the Mod Generation, the early Rock 'n Roll era, New Wave era (including the oldest school of rap), and the funky, soulful and progressiveness of the 1970s.

Best:
1965-1969
1970-1974
1990-1994
1975-1979
1980-1984

Worst:
1995-1999
2000-2004
2014-2018
2010-2014
1985-1989

Best
1. 1980-84
2. 1965-69
3. 1990-94
4. 1985-89
5. 1975-79

Worst
1. 2015-19
2. 1960-64
3. 2010-14
4. 2005-09
5. 1930-34

all I can say is that I like rock in the 80's  All years have their ups and downs and I cant remember whens what

After two months, I think I want to reorder my original list. Still focused on mainstream

Best:
1. 95-99: The very unapologetic and assumed cheesiness of pop music back then really resonates with me, and the edm scene was also a breeding ground for innovation. It's a bit hurt by a lousier rock canon but I can definitely get past that as it still produced many classic albums.
2. 00-04: Mostly identical to the late 90s, with a more redefined rock canon and a blossoming r&b-pop scene which I really enjoy. It's also a magnificent time for hip hop music.
3. 80-84: New wave is another thing I really enjoy, and the scene back then was very innovative and exciting. Add to that the lovely synthpop that came from it and you get pure musical bliss.
4. 75-79: A contrasting period where both disco and punk rock were popularized, both of which I happen to like a lot.
5. 65-69: I'm not a fan of psychedelic music as I already thought, and the cheesiness of pop was still fairly muted compared to later decades. Still, it is still a classic period for innovation for the jazz scene, being on its last legs, and rock too.

Worst:
1. 50-54: Abysmal period for pop music as it is mostly limited to crooned standards backed by orchestras or big bands which is so uncreative and personally unenjoyable. The jazz scene was also resting on its laurels by then after the bebop and cool jazz explosion of the late 40s.
2. 85-89: The burding creativity of new wave brushed aside for a heavy focus instead on testerone-heavy, faux-catchy glam metal and the most hackneyed adult contemporary ballads possible (both of which I like certainly much less than trap). Also super-action shows rawk power ballad music was also popular which is just ewwww.
At least out of this the underground scene was in a magnificent shape.
3. 90-94: Same reasons as the late 80s, adding to that dreary r&b ballads and annoying dance music and pop rap. At least the popularization of eurodance and gangsta rap around 93-94 put some excitement back in.
4. 05-09: The american scene was certainly not really in a good shape during this period mostly defined by an oversaturation of mediocre snap music. I still like a lot of the edm and r&b produced back then though. But some years like 2008 were nothing of merit I guess.
5. 60-64: The jazz scene was still in an explosive state of burden creativity caried over from the late 50s. But I still can't past the general dreariness of the pop music from back then, now completely free from rock & roll which added some well-needed new energy to the charts, while the British rock scene was still on its first steps.

2010-2014? Garbage? Lol okay then.