I always always more partial to Nickelodeon, for whatever reason. Cartoon Network shows had a certain "attitude" or something to them that I can't quite explain, but it's just not my thing. I enjoyed a lot of their shows, like Foster's Home for Imaginary Friends, Total Drama Island, Samurai Jack, Codename: Kids Next Door, Courage, both Star Wars shows, Hi Hi Puffy Ami Yumi (remember that one?), Camp Lazlo, and Teen Titans. And I think their programming has been killing it the past decade while Nickelodeon's been a dumpster fire, what with Adventure Time, Regular Show, and The Amazing World of Gumball (my favorite) being some of the best animated shows of all time already.
But overall in the decade I was primarily growing up in, Nickelodeon had more of my attention for sure.
Beware, as I spend too much time going down memory lane and Wikipedia.
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SpongeBob was an obvious heavy-hitter, especially in the seasons prior to the movie. No explanation needed, really.
Jimmy Neutron was also a favorite of mine, and it's a shame it never got an additional season. Apparently it would've been darker, actually ship Jimmy and Cindy, and have a bit more focus on continuity, which the show was always good at.
Fairly OddParents was also amazing, though it definitely went on too long. Very clever and creative show with great writing, and the really big special episodes were always a fun event.
My Life as a Teenage Robot was also my favorite for a while, especially after its TV movie that was really good. Really great and creative art style, along with good action, drama, and comedy.
Invader Zim just disturbed me when I was a kid, but I grew an appreciation for it in 2010 when they started rerunning it again (and it may or may not be a weird, primary source of inspiration for the aforementioned show I've written).
Danny Phantom was also pretty inspirational for having the balls to have a little more focus on continuity than the average show. Apparently the final and third season wasn't great on account of one of the key writers leaving, but I hardly remember it, so I can just pretend it doesn't exist.
Iron Man: Armored Adventures was also very formative for me, and is one of the most underrated super-hero TV shows there ever was. Thankfully, it barely survived being canceled before it could wrap up, and didn't crap the bed like Danny Phantom. The same can't be said for most other superhero shows.
Rugrats is always a classic, and I even liked All Grown Up.
KaBlam!/Action League Now, ChalkZone, Kappa Mikey, and Making Fiends were all fairly underrated as well. Apparently the Nicktoons channel wasn't as accessible, so a lot of these went missed, and that's unfortunate.
Hey Arnold, CatDog, and Catscratch were all fairly entertaining, but not really my thing.
Then there was that specific time period of their live action shows with Drake & Josh, Zoey 101, and Ned's Declassified School Survival Guide which were all more well-written and executed than any kids sitcom on those channels before or since. I'd still watch any of these today and sometimes do, but I see what they put on Nick/Disney nowadays, and it's almost unwatchable.
Speaking of, I preferred Nick & Nite to Adult Swim. I love watching Fresh Prince of Bel-Air, The Cosby Show, George Lopez, and other classics much more than I enjoy Aqua Teen Hunger Force or whatever else. Of course those are two totally separate demographics, and I wasn't really allowed to watch Adult Swim anyways, but even today that's still my preference.
Nickelodeon GAS was also amazing, and I'll never not be sad to see it gone. It was great seeing Double Dare, GUTS, Figure it Out, Legend of the Hidden Temple, Finders Keepers, and Scaredy Camp (which did NOT screw around). All that programming was reruns, but it was before my time, so it was new to me, and I loved it all. Shame stuff like that isn't popular nowadays.
But above all, there was this: