I agree with the Wesley Dodds Sandman one. His original get-up, for awhile they kind of shoehorned him into a costume, but I like it best with his suit, gun and gasmask. It's basic, but the mask gives off a pseudo-sci-fi noir look, and the fedora, can't forget that!
Before he kicked it, I liked Scott Lang's costume, where it had been updated and really made into his own version, not just him wearing Pym's old duds. It kept sort of the same look, but inverted the colors, and gave more trimming, and the helmet was updated and looked pretty sweet
There are others, but some of them have been said in this already. Velluto/Almond Black Panther, the original duds, but with Texeira's gold accoutrements and the cape.
Moon Knight, I liked Night Thrasher's new costume before he died, most of the Slingers, Force's new armor as seen in Iron Man around the time of Avengers Disassembled.
Spiderman
Yes, a classic choice, but only because it's true. The costume is the perfect balance between camp, cult and bad assery. And at least Spidey doesn't have uber-lame pointy ears on top of his mask.
Classic Venom
Very simple, yet effective.
Deathlok
He always looked cool. Very detailed, too.
Spawn
One of the reasons why he's my favorite character. He just looks so awesome. I want to touch him 💃
Classic Thor
Extremely distinct and recognizable. Although some of his later appearances are superior from an artistic standpoint, the classic one remains one of the coolest in comics.
The Punisher
Again, very simple, but oh so effective. Especially his more recent appearances.
Ultimate Wolverine
It's just plain badass, compared to the cheesy yellow costume. And I've always prefered to see Logan without the stupid mask anyway.
Batman is always at the top with the Year One costume.
Catwoman finally has a cool looking black costume.
Flash and Captain Marvel are similar. Yet cool
Superman and WW are simply Iconic.
Joker will never go out of style.
GL's uniforms stand out.
As for Marvel:
Ghost Rider is simply badass looking.
Punisher is just perfect.
Daredevil's red is the PERFECT costume.
Costumes that should really be reconsider:
Red Tornado, Spiderman, Captain America, Wolverine, Phanton Lady, and The Atom.