I've loved animation for ages, I actually prefer it to live action.
Batman the Animated Series
Cool and stylised, the 1990s Batman drew on Tim Burton's movies and expanded on the ideas that he had established. It proved to be the founding series of what went on to be what might be seen as the most accessible version of the DC universe.
A blast from my childhood, Duckula was a fun cartoon from the people who made Danger Mouse. Featuring a vegetarian vampire duck, who had a castle that could travel anywhere and who spent a fair amount of time dodging the machinations of the vampire hunter Von Goosewing.
Oban Star Racers
When an invading alien army is stopped by a mysterious intervention, humanity is baffled by the circumstances, the news that a great race to determine the fate of the galaxy will take place. The story follows a young girl who bluffs her way into the team from Earth, trying to get her father, the head of the team, to acknowledge her. Along the way, they have to deal with sabotage, mysteries, and weird aliens (some of them very strange).
This does feel like a bit of cheat, because the series is a French/Japanese collaboration.
Avatar: The Last Airbender
A beautifully designed and written series, Avatar takes place in a world inspired by Asian myths and styles. It follows a Japanese style nation's attempt to conquer the world, and the only Air Bender of the world acting to defeat them. It's a fun series with lots of beautiful world building and characterisation.
Probably the most fun version of the X-Men I've ever seen, X-Men Evolution is a teen based series, centring on a group of mutants going to High School and actually getting to be teenagers, rather than just soldiers in a war between mutants. It's well written and set out, and uses characters very well, allowing them to shine.