As has already been said, Samurai Champloo and Cowboy Bebop are a must. Fantastically written, acted etc.

Ouran High School Host Club is also a must. It's a shoujo comedy. Basically it involves several males who are in a club to flirt with women. the premise sounds fairly absurd I will admit, but the character development and sheer comedy. I was laughing very loudly many times.

Bleach and Naruto have already been said. Whilst they are heavily action based the characters are still very likeable.