Azumanga Daioh is one of the funniest anime I've ever seen, not to mention one of the cutest. :D Ranma 1/2 comes to a close second, although I've read more manga than I have seen the anime. I just remember that both those series were always funny no matter how many times I'd watch an episode.
For some reason, I find the first series of Yuugiou to be funny as well, although it's not even really a comedy. ._.;