The Replacements - At first I thought it was going to be one of those series that tried super hard to be funny, but once you kept watching, it was really clever when it wanted to be.
Dave The Barbarian - Wrote this one off at first for the same reasons with the Replacements, and this was actually hilarious as well.
Early 60's Hanna-Barbera Cartoons - Mainly the early stuff like the Flintstones, Jetsons, Top Cat, and Jonny Quest. I hated the first three things as a kid because I thought it was just dated without being a classic, like Looney Tunes. Not to mention, I hated HB for all of the crap they put out later in the 70s-80's, minus the original Scooby-Doo.
Part of me still feels way about it humor wise, but watching it a few years later, I really enjoy them on a nostalgic, simplistic level. I have to side with John K., the early stuff is really pleasing aesthetically. Jonny Quest , I originally overlooked this, but one day I just sat down and watched it and was completely blown away by it's sophistication and style.
Drawn Together- If gross out humor isn't your thing, then you probably wouldn't gravitate to this one, but I honestly haven't laughed that hard at a 00's cartoon..ever. Just completely insane and I loved every minute of it.
Lupin the 3rd - I caught this on adultswim out of bordem in 2003, and was completely entertained. Some people don't like it because of angular style, but it's actually one of the reasons it's my favorite anime to date. It's a shame it's not more popular stateside.
Also: One of the best theme songs ever.
Honorable Mentions: Brandy and Mr. Whiskers. the aforementioned Secret Saturdays, Archer, Venture Bros. and Jem.