I waited a lot before posting into this thread because most of the comics I like aren't on tapastic, I can't decide ONE favourite comedy comic and I can't really explain why I find them funnier.
Anyway, my favourites are Saturday Morning Breakfast Cereal, The Perry Bible Fellowship, and XKCD.
Probably I like them more because they are different from most of the others comedy comics and you sometimes have to think a bit to understand them. SMBC and XKCD have extremely clever jokes often about politics, Math and physics, TPBF is just genuinely brilliant, and none of these comics is afraid of making black humor.