I read a lot of webcomics on a daily basis (or whenever they update). It's hard for me to choose a favorite, because I love them all (I wouldn't take time in the mornings between waking up, checking the weather, and getting dressed to read them if I didn't) but I narrowed it down to xkcd, Sinfest, and Questionable Content. Since we're a book review blog, I'll choose the one I like best for storyline: QC.
The comic has been around for over a decade now (it launched in August 2003) and I've been reading it since at least 2008, but I'm pretty sure it was before then.
The characters are likable and seem like any average group of friends. On holidays or during other interruptions in regularly-scheduled comics, the creator - Jeph Jacques - uses Yelling Bird to (very profanely) amuse the readers during the lull.
I regularly use the phrase "Everything is ruined forever," which I got from this comic. I'm sure there are lots of other influences it has had on me, but they're so ingrained, I can't even think of them right now.
So, I actually really like webcomics. I think they're fun, they allow people to show off their amazing talents, without having to sell the idea or strip to a newspaper.
And, like Alex, there are a LOT that I'm a fan of (I really like xkcd also. It's hysterical.) I'm a huge fan of Cyanide & Happiness.
Sound like a certain co-blogger we all know and love?
However, my favorite webcomic, that I love to death, is actually one that has been finished for a few years now. Ozy and Millie was one of the first webcomics that I ever read. They're funny, and cute, and really, just always reminded me of my friends and I.