Back in 2003, there was a thread describing the pivotal song between the '70s and '80s in terms of "zeitelgeist", and just one 80s-90s mention: Groove is in the heart by Delite, but not much specific information is given.

Before you get your "Smells like Teen Spirit" hyped up for the song that really served as a pivotal point between the '80s and '90s, keep in mind that is only for rock, and other rock songs released from mid-1989 to mid-1991 also could have pivoted rock between the stereotypes of what the '80s and '90s are. For other genres, I'd say Aalanyah Miles' Black Velvet really had that transitional feel with the softer, non-synthesized AC sound so unlike 95% of all songs released since 1983, and paved the way toward Sheryl Crow-type pop. For dance, I'd say Technotronic's Pump Up The Jam; it really closes the door on Hi-NRG/Freestyle and points the way toward Eurodance. For R&B, I'd say Soul II Soul's Back To Life also really transitions to a '90s R&B sound as it is less glossy/cheesy than most R&B records during and before, as that song points the way toward Boyz II Men-type '90s vocal harmony and has elements of house music.

For the '90s-'00s; it's very brackish given that Nu Metal and Teen Pop were peaking in the middle of the real transition. Latin-type pop was also a very 1999-2000 thing. But I feel Destiny's Child's early hits paved the way for the Beyonce-type pop that dominated the '00s. But Faith Hill's There You'll Be ballad from the 2001 movie Pearl Harbor was the last Top 10 hit written by Diane Warren and thus the last that has that Celine Dionish sound and really closed the door for '90s-sounding ballads, as '00s ballads were really midtempos with a hip-hop flavor. Any ideas?

90's R&B

Ready Or Not-After 7(kinda played a part in starting the trend for what 90's R&B would sound like.)
Whip Appeal-Babyface(1989)-In my opinion Babyface invented the 90's R&B sound I mean just look at his production credits on his wikipedia page.


90's A/C
More Than Words-Extreme(1991)-yeah sounds like a hair metal ballad but sounds like a 90's A/C song too.
Release Me, You;re In Love, Hold On-Wilson Philips-All 3 songs sound like a perfect cross between the the late 80's and early 90's.
What I am(1989)-Edie Brickell and the New Bohiems(this song doesn't even sound like its from 1989 but it is.)
Fast Car(1988)-Tracy Chapman(again doesn't sound like a song that would come out of 1988 but its from that year.)
Luka-Suzanna Vega(1987)
I'll Always Love You(1988), Love Will Lead You Back(1990)-Taylor Dayne
I'll Still Believe-Brenda K. Starr(1988)(sounds like an underdeveloped Mariah Carey ballad but than again Mariah Carey was a back-up singer for Brenda K. Starr in 1988.) I just remembered Mariah covered this song in 1999 too.



90's Dance

Let The Beat Hit Em'-Lisa Liisa(1991)(sound like her but this sounds more 90's than 80's thus Lisa Lisa gets 90's cred.) This was a sign though the 80's sound was going out and the 90's sound was coming in.


90's Rap-

F*** Da Police-NWA(Think 90's Gangsta Rap)
Tic Tac Toe-Kyper(1990) Hey they sampled like Puff Daddy before Puff Daddy sampling "Owner Of A Lonely Heart" by "Yes".
Can't Touch This-MC Hammer(1990) Boy MC Hammer sampling like Puffy sampling "Superfreak" by "Rick James".

90's Alternative Rock-

All I Want-Toad The Wet Sprocket(1991)
Mountain Song-Jane's Addiction(1988)(this song was like 4-5 years ahead of its time.)
Epic-Faith No More(paved the way for those late 90's/early 00's Nu-metal bands.)
Man In the Box, Angry Chair-Alice In Chains(1990)
Head Like A Hole-Nine Inch Nails(1989 or 1990.)(This song was like 5 years ahead of its time.)

How about the Proclaimers - 500 Miles...it was in the a soundtrack of a movie made in 1995 and it came out in 1988.

90's-00's:

Butterfly by Crazytown.  ;D