I'm gonna go against the grain here and say my least favourite is Steve Rogers' costume he wears for the bulk of Winter Soldier, resembling his 'The Captain' costume he wore in the comics for a while. I actually prefer the cowled Captain America costume he had in Avengers. Yes, there was something silly about both the cowl and the bright primary colours, but for me it added to the sense that these were comic book characters translated to the silver screen. That image posted of him earlier on in this thread is a very unflattering one, at any rate. He looks a lot better in the actual battle scenes.
The second one would be Hawkeye. I just have a thing for masks. While I'd have preferred to have seen the 616 mask or a variation thereof (and no, he doesn't need the loincloth), I'd have settled for the Ultimates mask with the goggles.
About X-Men and the whole leather thing, even though I disliked it when I first saw it, because I associated X-Men with using costumes that are very character-specific (even though a lot of them return to the old blue-and-yellow/gold scheme). But in retrospect I've grown to appreciate the choice of the film studio; I get that the whole idea at the time was that the black leather suits work better for a team that's involved in a lot of covert-ops type missions, and maybe garish yellow, red, and sky blue costumes would hurt them.
Also, before the X-Men movies, attempts at translating superheroes' looks to the big screen in a credible way had often failed, with the exception of Batman (who wears a costume that's very subdued in terms of colour) and Superman (who is so iconic that people will look past it). Most recent actual successes of superhero adaptations at that point were Batman and Blade, who were both wearing darker, more practical costumes. I could see this influencing the choice in costume for the X-Men movies.
Then later, the MCU comes along, and proves that staying a fair bit closer to the comic books in terms of visual design also works.
Personally (and I think I'm with the majority, here) I thought the Milano, Dark Aster, and Nova Starfighter were all great designs.
