Surfer: gleaming simplicity.
Original Spider-Man / Parallax: best spins on the traditional spandex/gloves/boots/cape theme.
Aegis: cosmic-female gleaming simplicity.
Thor: his new look is perhaps the best costume upgrade I've ever seen.
I'm not sure which, but Nova has had some very cool armor/suit designs.
Photon (the one who looks wrapped in the night sky): simple, striking.
Hulk: you can't beat indestructible purple pants. About as natural a non-costume costume as you can get.