Artwork/design wise. Any hero, environment or villain. Personally I'd have to go with Dreamer's. The characters are great and the artwork is great. I really enjoy the different projections. Especially Tooth Fairy and Whipacorn.
Dreamer is really good, and I especially like its card back. However, I am also a big fan of both Scholar and Miss Information. They both have such a good mix of epic struggle and tongue-in-cheek humor.
Id have to say Nightmist. Playing her deck, more than any other, makes me feel like a paranormal investigator with sorcery that backlashes back to me. The art is all nice and dark and magic-y, which is just perfect.
I've said this before, and I'll say it again, The Scholar's deck is pretty good, for many of the reasons already mentioned on this thread. I also like healing as a way to do damage; I don't really know how that works thematically, but it probably involves balls of energy, and I am ALL for that.
Although he's not my favorite villian to play, Iron Legacy's art is so heartbreaking. I remember playing him solo and listening to Isreal's "Over the Rainbow/What a Wonderful World" and the dissonance I felt was pretty powerful. The discarded picture.... that's rough.
I love Argent's deck and I think Tachyon's fits her perfectly. For villian's, I'd have to go with IL, Plague Rat, and La Capitan.