EC: Snooze. He makes great EC and he doesn't take long to make it. Just give him a tileset and he'll make something great out of it.
Theme: Snooze. The theme in Dreaming.. is one of my favourites, and he has made lots of other levels with good and original themes.
Gameplay: blur. He makes levels that almost feel perfect. His levels usually have some secrets, and that's pretty fun.
Placement: EvilMike. Well, I don't know who else I could put here. Lots of people are good at placement, as it's the easiest part of JCS (in MP, not in SP)
Fastest Improver: PurpleJazz. He went from Easter Feasta to Sunken Realm in less than a year. That says something.