It's probably a bitter streak of mine, but I'd have a better chance at saying which webcomics I don't like and why rather than the other way around. But I'll give it a shot and see how far I ramble.
Well, Perry Bible Fellowship is pretty good. Concerned was funny. xkcd or whatnot is usually fairly clever. Doctor McNinja is pretty well done, with excellent artwork now, much improved over when it first began. Pretty clever and funny, to boot. I like the references dropped, and when the zombies did that Thriller dance I laughed.
PVP's too random, but the art's pretty alright and definitely improved. VGCats seems like a poor attempt to copy this, but with (get this!) cats! Sorry, not wowed.
I cannot stand CTRL+ALT+DEL or Megatokyo. Inane, with poor choice of stylization and art movement, and dull plot segments in both. CAD also seems to be in poor taste half the time, and it being intentional doesn't help. The creator of CAD apparently ran into some trouble involving him, ah, exposing himself to a minor along with his being a pompous hosebag (Though that's probably not something he can change). Hmm. The creator of Megatokyo, who probably goes by some Japanese moniker I can't be bothered to memorize all 12 syllables of so I'll just call him Freddy, apparently ripped off his friend Largo.
I could make some kind of weight based attack on ol' Freddy, like calling him, "Large-o" in being similar to his former underling (That's their relationship, right?) Largo's name, but I won't. Interestingly, Largo's leaving is when Megatokyo stopped being tolerable and became some sort of self-insert Japanese comic wannabe webcomic.
Ack, see what I've gone and done? Can't comment once on webcomics without bringing up some of the more popular shills and hacks.