While I still think that webcomics as a medium still has to mature a bit, before reaching a true masterpiece level of quality, on the same level as say, Watchmen is for western comics or Homunculus is for manga
I did find this Tapas content to be truly a cut above:
Al Khemet, while not a "comic" per se, at least not a narrative comic, is a brilliant showcase of art, there are reoccuring visual themes, that help make it slightly more tied together than an art gallery, but don't expect much in terms of narrative.
Sin is a shonen manga style webcomic, it still has a lot to develop and it's only now starting to reveal the complexities of it's characters and arcs, but it's a quite promising comic.
Crystallium Saga is quite short, but it's a good example of showing instead of telling, it's a collection of sci-fi short stories. The art is quite good also. It's still not long enough to give a definitive rating of quality, but it's still worth a mention.
River and the Parallel worlds is still quite new, so I can't quite give it a rating, but it's got an intriguing premise and solid art, which alone merits it a mention for a solid series.