First of course must be Nedroid. Oh my, this one is wonderful.
Second would be the aforementioned Hark! a Vagrant. It blends dry humor with pretty good drawings. She injects a lot of personality, which is difficult when your characters are different each time. Well done.
In third place is Dinosaur Comics. All the pictures are the same every time. This pictorial laziness was a reason I used to hate xkcd. But like xkcd, it has nevertheless won me over. (bonus points whenever he plays with his own format by introducing God or raccoon and cephalopod neighbors.)