Even though I am not a big Ironman fan or Tony Stark fan, but if were talking about cool costumes than my vote goes to Ironman and War Machine. Those two guys always had cool designs. It was those designs that made me want to read the comics.

Second place would have to go to Wolverine's costume Circa 1992-93, when he wore his gold and blue strip uniform with blue armor shoulder pads, that was freakin awesome. 

Third place would have to be Cyclops costume Circa 1992-94. It just looked cool him exposing his hair and the presece of his comlink he wore like a vest on his chest.

For the ladies.
Catwoman's costume Circa 1994 that costume looked hot with her hair exposed and without the goggles she wears now.
Susan Storm Circa 1993 when Reed Richards was thought to be dead, her skimpy costume was hot, but it designed very well so that her tits don't fall out, ( Makes me wonder how Starfire and Blackcat fight crime without anything popping out)lol.

The best costumes today would have to be Fairchild and She-Hulk's body suit, its sexy but at the same its made for fitness. So both ladies can workout in it and look good. If its good enough to work out in, then its good enough to fight crime in.

I know the question asked for one, but I just think there is more than one cool costume.