From back in the day, It's a pretty hard choice between a bunch of Cartoon Network's old lineup from the 2000's; Samurai Jack, Codename: Kids Next Door, The Grim Adventures of Billy and Mandy, Teen Titans, Ed Edd n Eddy, Fosters Home for Imaginary Friends, etc. I honestly couldn't pick just one. It's a pretty legendary time period in animation history without getting into Anime, and Nickelodeon even got their hat in the ring with shows like Spongebob and Avatar.
These days I don't really watch a whole lot, I mostly spend what free time I have gaming or chatting with friends, and when I'm not doing that then I'm probably browsing the forums here or on Steam.