Lastly, these aren't "the best". They're "the ones I liked the most", of a tiny sample of the thousands of web comics in existence. In the absence of a universally accepted measurement of the quality of a web comic, this is the best I can come up with.
In the end I've had to go at this job the following way:
* No comics written by friends (I'm probably inherently biased for them because of them being written by friends, which also means I'm probably too harsh on them when I try to review them because I try to counteract that bias in their favour). There's three in that category, and they're all recommended: Allos (start at the beginning); Intermittent Tangents (the best story starts here); and Kismetropolis (start at the beginning).
* Choose five favourite "strip" comics, and five favourite "page" comics. (Sometimes I feel in the mood to read one type and not the other; I've done this to try to balance however it is I'm feeling about reading them).
* Comics that are still active and still up to their standard (for example, I've had to regretfully exclude "Black Tapestries", which hasn't been updated for months; and "Sluggy Freelance", which has really gone off the boil in the last year or so as far as appealing to my taste goes).
So, without any further ado:
The Cyantian Chronicles. This one is probably a bit borderline as to which category it goes into, but it seems to do the strip-sized comics more often than the page-sized comics, so it's going here. Seems to have been nibbling around the edges of a huge story for a while, and looks like it's heading for the final run-up. Basically, an alien civilisation (which may or may not have been genetically engineered from Earth stock) is preparing to make First Contact. They've already been terraforming Mars as a token of goodwill to the Earth authorities. Said Earth authorities are taking everything the wrong way, and think there's an alien invasion in the offing. However, between the earlier comics and the ones showing this planned contact, there's been a catastrophic war which it's suspected many of the characters in the "lead-up-to-war" comic ("Akaelae") won't survive.
Freefall. Seriously philosophical comic dealing with the idea of artificial intelligences of various types, and questions of how much freedom of thought or action they would have. At the same time, it's a continuing science-fiction story, and a strip with good gags.
Get Mediaeval. Regular as clockwork, every day, and with a minimum of filler, this would have to be the most reliable comic on my reading list. The story's very good (and it's not going to go downhill - it's got a definite finishing point, which, much to my disappointment, it will reach in the next few weeks) and, while the aliens stranded in mediaeval Earth aren't really all that alien, the mediaeval Earthlings are believable. The ugly parts of mediaeval life aren't hidden out of sight, either, although they're not dwelled upon, in keeping with the wry but optimistic tone of the story.
Suburban Jungle. One of the most stylish and professional looking strip format web-comics out there. Having characters (and therefore, obviously, a writer) with a decent knowledge of `eighties music is a bonus. The only thing I'm lacking for a greater appreciation of this strip is anything more than a vague recollection of "The Prisoner".
User Friendly. Probably too geeky for its own good but still has a place in my heart as the web comic that first drew my attention to the genre all those years ago.
Digger. The story's interesting, and reliably weird; and the artwork is unique, having what is almost an art nouveau quality to it. The only flaw in this comic is that only the first half of the storyline (and whatever the current page happens to be) are available except to paid subscribers.
Paradigm Shift. A slow-moving and finely detailed (both in the writing and in the art) police / mystery story. The pieces are still moving into place, but I have high hopes for the future of this story.
Purgatory Tower. The concept here is as huge as the tower the inmates are climbing. But concept alone isn't sufficient to make a comic good. In this case, the characters, story and artwork are all high-quality and make for an intriguing read.
Questionable Content. The music may be of a generation later than mine, but the story being told is universal and the characters manage to be at the same time quite bizarre and yet still "I-know-someone-like-that". This one really needs to be read from the start; the first hundred or so pages aren't all that funny (or all that well drawn), but this is a case where perseverance pays off; once you know the history of the characters, so many odd statements in the later pages of the comic become hilarious.
The Wandering Ones. This comic is threatening to make "old-fashioned" cool again. With a story reminiscent of Rudyard Kipling (but without the jingoism), and line art reminiscent of twentieth-century adventure-story illustrations, this story of a wilderness civilisation in a post-catastrophic America of the (now) not too distant future is simply a cracking good read.