The first basis for any webcomic is posting frequency. Webcomics post as often as the artist and/or publisher gets the finished (sometimes unfinished) product live on their website. Most webcomics make a promise or guarantee to their readers that they will publish X times per week or on specific days. Unfortunately this often results in some terrible cases of "filler" content. Filler content is when the artist/publisher makes some form of obnoxious short quip. An example of a webcomic with a LOT of filler is School Bites. School Bites is a webcomic about a vampire teenager who eats cupcakes and has a human boyfriend. While the plot itself is questionable the artist often uses a mass amount of filler about her cat. I find it distasteful as, if I wanted to get to know the artist and what is happening in their life, there is usually a blog of sorts below the comic to keep up with artists, writers, and publishers alike.
What you want in a webcomic is frequent and consistent updates with little to no filler. The best webcomic in my opinion for multiple categories is Least I Could Do. The focus here is on filler and lack thereof and Least I Could Do not only updates daily but does so without any clear filler. The only thing that could be considered filler from this webcomic is the Least I Could Do Beginnings which offer a glimpse at the childhood of the protagonist Rayne Summers.
Least I Could Do is easily the best webcomic on the internet for multiple reasons however I shall simply summarize that while though the adult humor is more than a little thick the plot and soft moments that Least I Could Do provides makes this a must read.
Good plot is as equally important to the webcomic selection as the posting consistency or frequency. One such comic is Amya, which has very good posting frequency but even better plot. Plot helps drive the development of characters, it helps them grow, it helps them mold into who they become throughout the life of the webcomic. A comic with really great plot can even overcome being riddled with filler such is the case with Flipside, which follows the stories protagonist Maytag and her girlfriend, friends, enemies, and love interests. The character development not just of Maytag but of all of the characters of Flipside are what shoots this to number one on my list in terms of plot line.
The best webcomic in my opinion in terms of plot, frequency, and overall content is Goblins. Goblins is the story of a group of goblins that decide that they're tired of being hack meat for low level adventurers and choose to be adventurers themselves. The story follows the humorous misadventures that they find themselves tangled up in constantly. As of late we've been following a rather exciting plot offshoot involving an entertaining trio. If you're looking for a solid, entertaining, and sometimes bloody webcomic then I recommend Goblins as a primary selection choice.
Finally I would like to talk about one of my favorite webcomics, Manly Guys Doing Manly Things. Rather than get into how great I feel the plot is, how nice the artwork is, and everything else I'll just leave you with several of my favorite strips. You should at the very least read these strips here: The Saga of Jared Kowalski, the worst Pokemon Trainer ever.
"But mom.... There's wild Pokemon in the tall grass!"