As I already mentioned in the title, this is likely to be a controversial or unpopular opinion I'm stating here, but I honestly think in terms of quality, the era of Nickelodeon that came after the Klaspy Csupo era (roughly June 2004 to September 2007) is better than the Klaspy Csupo era itself.
Perhaps it's because the 2004-2007 era was during the peak of my childhood, and the Klaspy Csupo era I really only caught the tail end of, but even when rewatching these shows and looking at them without nostalgic bias, my opinion still stands.

I'm definitely not suggesting 2004-2007 as the very best era for Nickelodeon, that title obviously goes to the Golden Age (1991-1997/8), both for the Nicktoons and the Nickcoms in that era, no era before or since even comes close in terms of quality. But if I were to choose a runner-upper, I'd chose the 2004-2007 era rather than the Klaspy Csupo era.

The Nickcoms of the Klaspy Csupo era were rather mediocre, especially compared to the ones from the Golden Age of Nickelodeon. You still hear older Millennials discuss Golden Age Nickcoms such as Clarissa Explains it All, Salute Your Shorts, Are You Afraid of the Dark, Legends of the Hidden Temple, Adventures of Pete & Pete, All That, Secret World of Alex Mack, and Kenan & Kel all the time when they discuss television from their youth, but how many core Millennials do you honestly hear talking about Klaspy Csupo era Nickcoms such as Journey of Allen Strange, 100 Deeds for Eddie McDowd, Caitlyn's Way, The Brothers Garcia, Noah Knows Best, Taina, or even the Nick Cannon Show? You almost never see or hear it, and for good reason, these shows were rather dull and unconvincing, and definitely paled in comparison to their Golden Age counterparts.
Sure, there are exceptions, such as The Amanda Show and the latter half of Kenan & Kel, as well as the tail end of the original All That, but other than that, it was nothing for Nickelodeon to gloat over. Speaking of All That, they brought that show back in 2002 following a hiatus in 2000, but with an entirely new cast, and it was awful, to say the least.

As for the Nicktoons of the Klaspy Csupo era, the picture is definitely better, but that's not saying much. There were definitely quality Nicktoons in this era such as Hey Arnold, Angry Beavers, SpongeBob SquarePants, Invader Zim, as well as the early episodes of Fairly OddParents, ChalkZone, and My Life as a Teenage Robot, but the other Nicktoons of this era have not stood the test of time, especially compared to their Golden Age counterparts such as Ren & Stimpy, Rocko's Modern Life, and Aah! Real Monsters.
Many older Millennials point to CatDog as the start of Nickelodeon's downfall, and it's not hard to see why. I used to love that show when I was younger, but looking back, I now find most episodes of that show to be rather predictable and repetitive, and it's a shame too, the idea of a show with a conjoined cat and dog is brilliant, yet it just wasn't executed as well as it should've been. Most older Millennials hated Rocket Power, and rightfully so because the show itself was awful, and solely made to ride off of the Y2K Era popularity of extreme sports. Then you had As Told By Ginger, which was a terrible slice of life TV show based solely off of cliche girl drama that should honestly be reserved for the Nickcoms, and later you had Jimmy Neutron, which may have been good at the time, but it's honestly aged very poorly.
Going back, you had Rugrats go way downhill in this era following the theatrical movie, in which they introduced Baby Dil Pickles, who is honestly the Rugrats equivalent of Poof from Fairly OddParents. The Wild Thornberries was okay, I watched a few episodes with my sister when I was younger, but never really liked it, and looking back, I find it to be very mediocre, maybe because it's not my kind of show, but I also think the show was trying to ride off of Steve Irwin/naturalist popularity going on in the Y2K era, which I think was executed rather poorly.
Overall, this was a decent era for Nicktoons, but it was definitely a large step down in terms of quality from the Golden Age Nicktoons.

The 2004-2007 era, on the other hand, I feel had the better quality overall, both for Nicktoons and Nickcoms.
This was definitely the Golden Age for TeenNick, no doubt about that. Drake & Josh was definitely the top dog, but you also had Ned's Declassified, Unfabulous, and Zoey 101 debut in this era as well, sure the latter two may be more for the female audience, but all four of these shows are of much higher quality than nearly all of their Klaspy Csupo era counterparts (sole exception being Kenan & Kel), and you still hear younger Millennials and older Zeds reminiscing about these shows to this day.
As for Nicktoons, you had Danny Phantom and Avatar: The Last Airbender, as well as the peak of the Fairly OddParents and My Life as a Teenage Robot, these four shows were without a doubt some of the greatest Nicktoons to ever grace the network, even rivalling ones from the Golden Age.
You also had Catscratch, later episodes of ChalkZone, and even SpongeBob I honestly feel was still a quality show, up until they did that Atlantis special.

Sure, the Klaspy Csupo era did have Slime Time Live, as well as some quality game shows like Figure It Out and Double Dare 2000, and the 2004-2007 era did have a few awful shows that came and went such as The X's, My Dad the Rock Star, and Mr. Meaty, for reasons like those I could see the argument for the Klaspy Csupo era being better.
But overall, I still feel the 2004-2007 era is better. It was definitely better for Nickcoms, and I honestly even feel it was better for Nicktoons as well, as they felt more "edgy" and "badass" in the 2004-2007 era, while in the Klaspy Csupo era you had what I felt like was an overabundance of realistic slice of life TV shows that all felt too similar to one another, likely because most of those shows were made by Arlene Klasky and Gabor Csupo, hence why the era is called the Klaspy Csupo era, since this is when those shows were most numerous on the network.

What is your opinion regarding this?

Interesting points. I tend to lump Nick shows differently from you though. I consider 1998-2005 to be the silver age of Nick, 2005-2009 the bronze age, with Catdog being the first silver age show to premiere, and Avatar being the last silver age show to premiere (though its quality peaked in the bronze age with Book 2). I'd oftentimes heard many people on forums back in the late 2000s/early 2010s go on and on about how Avatar was Nick's "last great show" or "last real show", plus it was the last show to premiere when Nick Studios was open.

I tended to think of Catscratch or The Xs as being the premieres of the bronze age; they kinda had a different "feel" to me when they premiered, like we had crossed into a new Nick era that lacked the "soul" of before, maybe because they were the first to premiere after Nick Studios was shut down, or maybe because I was just getting older, IDK. When I saw the commercials for Catscratch they made it seem like it would be crazy awesome, then I watched the actual show and every joke and setup was completely stock/routine, not like the commercials. The Xs I felt was rather bland except for Glowface who was awesome. By 2005 I was getting a sinking feeling for Nick.

Interesting points. I tend to lump Nick shows differently from you though. I consider 1998-2005 to be the silver age of Nick, 2005-2009 the bronze age, with Catdog being the first silver age show to premiere, and Avatar being the last silver age show to premiere (though its quality peaked in the bronze age with Book 2). I'd oftentimes heard many people on forums back in the late 2000s/early 2010s go on and on about how Avatar was Nick's "last great show" or "last real show", plus it was the last show to premiere when Nick Studios was open.

I tended to think of Catscratch or The Xs as being the premieres of the bronze age; they kinda had a different "feel" to me when they premiered, like we had crossed into a new Nick era that lacked the "soul" of before, maybe because they were the first to premiere after Nick Studios was shut down, or maybe because I was just getting older, IDK. When I saw the commercials for Catscratch they made it seem like it would be crazy awesome, then I watched the actual show and every joke and setup was completely stock/routine, not like the commercials. The Xs I felt was rather bland except for Glowface who was awesome. By 2005 I was getting a sinking feeling for Nick.


I also believe that "Good Neighbors", which was the first noticeably bad Spongebob episode, aired in 2005.

When it comes to Nickelodeon's eras, this is probably how it is broken down:

1977-1979: Gestation Age (when Nickelodeon was known as "Pinwheel")
1979-1986: Pinwheel Age (start of Nickelodeon using the Nickelodeon name to the premiere of Double Dare)
1986-1991: Pre-Golden Age (premiere of Double Dare to the start of Nicktoons)
1991-1998: Golden Age (start of Nicktoons to premiere of CatDog)
1998-2005: Silver Age (premiere of CatDog to the closing of Nickelodeon Studios)
2005-2009: Bronze Age (Closing of Nickelodeon Studios to the time the logo changed)
2009-present: New logo Age (since the change of the logo)