My three personal favorites are her past/present, depending on your perspective, Marvel now look, her New Xmen look, and her good old green phoenix costume
@dayvid3: bah-hahahaha! that's pretty good..
@jojo111: I can't get down with anything to do with the Phoenix, just can't do it.
I do think most characters can pull off the school uniform, but I always think she looks weird with the original mask. Although I do like versions of it that have her hair coming out the top, or that just forgo the mask altogether.
But my all time favourites would probably be a tie between this X-factor outfit
and her Morrison era look, depending on the tone of the story or what she's doing, I guess.
I dunno if you guys know this, but it's an interesting piece of trivia that the white Phoenix costume was actually the original design. It was rejected in favour of the classic green because, due to the way comics were printed in the late 70's, the art editors realized that you'd be able to read the next page through her costume.
I've actually always thought that would have been a cool effect, given the nature of her powers, but I guess it would've seemed a bit random if the next page was an add for X-ray specs, fruit pies, or a "realistic" human skeleton -only 75 cents!
I Love her original Age of Apocalypse costume, along with her blue X-Factor outfit, her green Phoenix outfit (although I tend to like the way it looked on the animated series more so than the comic for some reason) and her current All-New X-Men outfit.