I prefer functional, especially if it looks "natural" in context (eg, Surfer's glaze, Hulk's pants). I also generally prefer simpler costumes.
I do like Spider-Man's traditional garb (as far as spandex goes). But one of the best of the more traditional superhero suits is Bale's Batman's suit. It looks cool and realistic in live action. No primary-color girlieman tights or outer-undies.