If it's Champloo, then Cowboy Bebop's an absolute must (not that it isn't under any other circumstances). lol
Steins;Gate's another definite, Eden of the East is short and sweet (the films are necessary), Durarara!! and Baccano! are great, too. Wolf's Rain is another great one (watch for the four recap episodes, though), as is The Melancholy of Haruhi Suzumiya (the film's even better if you end up enjoying the series), and if you can appreciate what it's trying to do, Mushishi's also amazing.