I tend to visualise Harry as short, slender, and androgynous/effeminate of looks - highly influenced by my taste in fic, btw xD - and Snape as tall and thin, but sort of whipcord-strong, with a rather pronounced Aquiline nose, sharp cheekbones, high forehead, thin lips... Hahaha. This usually comes out pretty well in my art - but not necessarily the same way each time, because apparently consistency is my fail-point. xD
I actually don't have many favourite artists in-fandom, because I actively avoid looking at pictures most of the time. This is in no way a reflection of the general quality of art; rather, it is a reflection on the fact that a shared 56k dial-up is royally sucktastic. (I did try to look at all the art for Back to Hogwarts, though! ...Which reminds me that I should repost that art to my journal..) Buutttt umbrus has some pretty stuff, and accioslash for the funnies.. veridian_dair also does pretty work. Hm. Usually I like a lot of art and respect the artist's visualisation of Harry and Snape, even when radically different from my own. When I don't like something, it's generally more of a technique or anatomy-whoopsie that is glaringly obvious to me even if I couldn't necessarily do better at whatever it was xD - many years of self-taught observation so I can fix my own art has made this an automatic reflex. ^^;;; (If it helps any, I do the exact same thing with fic! *sheepish grin*)