One that I would recommend is Detective Academy Q. It's focused on a class of five students from an exclusive detective school, and their adventures as the solve cases and eventually discover a secret organization that is the Headmaster's, the founder of the school, nemesis. Unfortunately, it ends with some questions unanswered as it was canceled before it could wrap everything up, and it hasn't been dubbed yet - I don't know how you feel about watching subbed anime.
For some dubbed ones, I would recommend the following:
Shoujo:
Ouran High School Host Club
Fruits Basket
The World God Only Knows
Shonen:
Yu Yu Hakusho
Sword Art Online (this one is going to start appearing on Toonami, if you get Cartoon Network)
InuYasha
MAR Awakens Romance
Detective Conan/Case Closed (It's up to 702 episodes, 16 movies, 12 OVAs, and it's still going. ^^; I've only seen up to the 60s in episodes and the sixth movie, so I can't vouch for the entire series.)
I'd also like to put my vote in for Fullmetal Alchemist: Brotherhood and Durarara!!, and add that the original Fullmetal Alchemist is also good (some might argue that one is superior to the other, but I love them both equally). I can't think of any more right now, but I can try and find a summary for any of these if you want one.
Edit: Just remembered Ranma 1/2. It's a nice little comedy. Also, I might have mislabeled one of these under the wrong category (as in they belong in an entirely different one - I don't think I got any mixed up between the two).