Best or worst based on what criteria?
Faithfulness to the original comic book costume? Overall just great look?
BEST
I've always thought The Crow was the best comic based movie I've seen, that said the costume Brandon Lee wore was pretty faithful to the original series AND very plausible - if not plain.
Iron Man and the Maguire Spider-Man did what they did very well.. IM espcially well considering what could have been very, very terrible. I'll add Kick-Ass to that, as it adapts itself pretty well from page to screen - even if it's not a show stopper-wow-look-at-me costume. I'll add Rorschach and the Comedian from Watchmen as comic book costumes that might be understated but carried out well onscreen.
Thankfully, super-hero costumes seem to be much easier to produce in comic fashion now a days and there are a plethora of good looking costumes.
WORST
The Singer Superman costume is just plain bad and misses the mark on an iconic costume. But then again, the whole Singer Superman just missed the mark.
The X-Men costumes - also from the Singer films. They barely qualify as costumes, do they? When they don the black pleather things, I'd just as well see them in street clothes.
Doctor Doom in the FF movie. I didn't like the movie, his costume was just as bad.
Spider-man's Venom/black costume. Just a poor idea turning the existing costume black - just lazy.
WORRIED
The new Spider-Man and Green Lantern costumes are causing me great concern. I am hoping for the best, but early looks have me worried.
I'll add Captain America to that last, although I am still optimisic. Some of the images looks a bit too baggy and more than a little busy... and some of them look very nice.