In order, that I read religiously:
Order of the Stick -- Rich's crankiness aside, this is the tops right now.
xkcd -- RM's stick figures threaten to be the alpha and omega of webcomix. They are certainly the best gateway drug.
Goats -- Insanity for almost as long as Sluggy. Chronicling the end of the mulitiverse. And a fish that lives in beer.
Sluggy Freelance -- One of the longest running webcomix and of those that old (with a continual story line) my favorite. Warning, may branch off into seemingly tangential storylines for 6 months or more.
Erfworld -- Rob and Jamie do a nearly perfect job.
Questionable Content -- Indie comix and silliness, shaken up with the Daft Punk 9000.
Least I Could Do -- Every guy knows or is or wants to be Rayne.
PartiallyClips -- Rob's older comic. Like a mix of Dinosaur Comics and Married to the Sea -- or maybe they got their ideas from Rob. . .
Penny Arcade -- Always good, always snarky -- oh and they've done the most to branch out of webcomix into the real world -- PAX anyone?
Fragile Gravity -- Apparently not many people read this wonderful comic. There's a stoat. And penguins. Waark.
pictures for sad children -- Long John's Silvers is an excellent place for the whole family!
Hark! A Vagrant -- Did you do well enough in History class to think these are funny?
nataliedee --The last four are all from Drew and Natalie Dee. Sort of the Far Sides of the pixel world.
Married to the SeaToothpaste for DinnerSuperPoop