Achewood
In my opinion, there is no finer web comic than Achewood. You want subtlety of art and a one-of-a-kind worldview? This is it.
Penny Arcade
The classic. Gabe and Tycho are true pioneers in the world of web comics, and few have done more to expand the reach and influence of the medium. Plus they're funny.
Diesel Sweeties
Like PVP, DS has been coasting for some time. But it is worth reading, nonetheless.
PVP
Yes, I do recommend it; its very popularity is what recommends it. It is the ultimate example that web comics can have mainstream appeal, and shouldn't be disregarded by the industry just because they're web comics.