Probably my favorite comic, but for one it was retired twelve years ago (and I've probably read the whole corpus) and for another its author, Gary Larson, doesn't like it when his drawings find their way on the Web (a wasted worry, I take it from the numerous exemples Google returns), is The Far Side. It is hard to characterize The Far Side as it is difficult to characterize any form of humor, but we can describe it as often absurd and improbable, sometimes slightly “sick”, but even then, in its own way, “cute”; and its depiction of that noble animal, the cow (Bos taurus), is bordering on genius of the greatest kind. For some reason, The Far Side seems to be a favorite of scientists and science nerds.
But anyway, given that I've read all The Far Side and that it isn't online, what else can I suggest? I'll give a few examples and, if possible, link to a couple of strips that I consider representative of the comic as a whole or of its form of humor.
One that I read every day and which sometimes comes close to Gary Larson's sick-but-brilliant style is Wulffmorgenthaler. I think the 2007-08-10 or 2007-12-26 strips give a good idea of what the best of Wulffmorgenthaler can be; the 2007-02-07 strip shows how it can be Garylarsonesque; as for the offensive-but-funny part, that of 2007-01-03 is rather typical.
Next I turn to The Perry Bible Fellowship (don't try to make sense of the name: if you really need to know, Wikipedia has an explanation), which is published much more erratically. Its whimsical and absurd humor, often morbid even when drawn in smiley-face-style, can be upsetting, but at it's best it is also very reminiscent of The Far Side. The One More Day strip is rather representative of the comic as a whole and Book World of its more sinister aspects, while Zuthulus Resurrection and Billy the Bunny are typical of a (viewpoint-shift?) form of humor which is often found in PBF. Unfortunately, the most recent strips have, in my opinion, not been up to par.
The Order of the Stick is a very different kind of beast, because it has a continued plot (and a rather subtle one, too): you probably won't enjoy it immediately if you just start reading it at a random point (although there are, of course, some pages which can be read independently); and the fact that the current issue as I write is #489 (that's 489 pages, not one-line strips) may seem a bit intimating (it took me a good part of a week, this summer, to catch it all from the start). But I think it's well worth trying to hop on (you don't have to start from #1, but at least read the cast)—if you like (D&D-style) roleplaying fantasy worlds, that is. The best comparison I can think of is with Terry Pratchett's novels, even though I'm much of a Pratchett fan. I guess page #100 is rather typical, and doesn't require much background to undestand.
There are a few geeky incontournables I guess
I should
mention: Piled
Higher and Deeper
(aka PhD-comics) is one for all grad
students
(this
strip is rather typical, if you need an
example). xkcd is another
for all math/computer geeks. I don't think either is as good in
general as the previous ones I mentioned: but the numerous
private jokes make them sometimes funnier for the targeted audience
(and, let's admit it, this strip
or this one are quite good).
But now, last but not least, I give you my favorite of all
webcomics: the (semi-weekly) unfeasible adventures
of Beaver and
Steve. I can't even begin to explain how this strange
mixture of absurdity, silliness and cuteness can produce such a (for
me) hilarous effect—and I'm perfectly sure that many people will
find Beaver and Steve entirely unfunny—but part of
the explanation is certainly
that the author
is British. The comic is generally issued in self-standing
pages: this
one, this
one
or this
one are typical of these; however, some of the funniest parts are
found in mini-stories of various lengths spanning more than one issue:
comics 159
& 160
are an example of this, but the best is probably
the Revenge
of the Shoe Goblin storyline (starting at comic 208).
Another interesting feature of Beaver and Steve is the
occasional guest comic
: while I generally don't find them as
funny as the original (James Turner) comics, some of them are also
very good
(this
one for example).