I’m becoming quite partial to Stripperella 
For male superheroes, or the one I’d want, Spawn.
Gotta go with Captain Marvel for coolest costume ever. The Big Red Cheese sports the quintessential iconic superpower emblem on his chest (nothing says “power of the gods” like a big-ass lightning bolt) and a jaunty over-the-shoulder bullfighting cape, while managing to completely avoid that hoariest of all superhero costume glitches, the “underpants on the outside” look. Actually, Captain Marvel Jr.'s costume may have been just a trifle cooler, because it incorporated all three primary colors. And, now that I think about it, Mary Marvel’s costume was probably the coolest of all three, because it had a girl in it. A superpowered girl, no less! A flying superpowered girl, who wears a skirt! Shazam!
Okay, maybe not.
Kid Flash (His second – yellow and red with his hair showing).
The original Green Lantern, especially because of its origin (it was an old costume from a costume party he had lying around).
Superman’s costume is the real classic, though.
The current JL cartoon’s Green Lantern has one of the best costumes ever becase it follows two classic concepts of art.
#1 A color scheme of one basic color plus neutral shades of black,white, and grey.
#2 an outfit designed to draw attention to the face and hands, the most expressive parts of the body.
Definitely the Golden Age Sandman. The Mystery Theatre version with the trenchcoat, though.
Those cutelittlw booties with the wings on them…and, those trunks…say, how did Robin avoid catching cold?
Kevin Matchstick (of Mage). Not only the cool, white-on-black lightning bolt t-shirt, but oddly enough, I remember in one part of the original series, Kevin goes to a store and buys a -pack- of them as if they were just normal white t-shirts. Gotta love it. Besides, the guy could accesorise! (Who wouldn’t want a baseball bat of power?!)
As someone who loathes costumes on characters generally, I reluctantly pick Deathstroke the Terminator, the DC protagonist who had his own title and fought the Titans.
Chainmail armour, a full- face mask (black on one side because he was blind in one eye and good enough not to care of anyone knew about it), and otherwise lots of weapons.
Practical, and it looked good.
Expanding the topic a fraction to include supervillains, I think Hunter Rose (Grendel) has far and away the best costume.
http://www.darkhorse.com/products/zones/z_grendel/index.html
Superman. Its’ so well-recognized and still cool after all these years. I hate the lack of face covering though. Oh well, nothing’s perfect.
If we are including supervillians than Mojo Jojo. I love the sight gag with with cape always blowing even indoors (in the beginning of the movie he wore a ragged towel around his shoulders that also blew in the non-existant wind).
Samaritan, from “Astro City”. Very simple design, relatively subdued colors, and it’s technically not even a costume, it’s just the outfit he was wearing when he was sent back in time. Very fitting for a guy too busy to worry about little things like clothes.
Emma Peel’s jumpsuits. Rowr!
Mon-El.
Two of my favorites have been named already:
Golden Age Sandman - pure class and a real sense of drama with a three-piece suit, trench coat, fedora, and GAS MASK.
Hunter Rose (Grendel) - all black with white gloves and boots, sort of a baggy feel (not skin-tight), and the greatest mask design of all time: skeletal eyes dripping teardrops and a skeletal NOSE, white on black, purely emotionless, always looking sinister. The two flowing straps are a nice touch too.
Two more:
The Rocketeer - pure retro pulpy goodness, with the bomber jacket, jodhpurs, and tall boots, plus the old-school jet pack and metal helmet with the grate over the mouth and that sweet aerodynamic FIN.
Reuben Flagg and the Plexus Rangers (from American Flagg!) - the greatest patriotic comic book costume ever, and these are uniforms! Red, white, and blue (with some black), shiny (leather, maybe?), high boots, a logo off to one side on a “suit jacket” with lapels, and even a necktie! I’d probably wear this in real life if it existed. Too cool.
Ditto Sandman (Gasmask == niftitude), Mon-El/M’onel/Valor, The Marvel Family and Grendel.
I like the X-Men’s uniforms in Evolution.
Spidey’s black costume - when it had him, not after it became Venom.
I liked Ice’s. (Of the JLA.)
Green Lantern, from Hal Jordan on is always good, although Kyle’s is an improvement.
Gambit (gotta love that cajun feel)
Archangel
Doc Ock from the new spiderman movie (he has a black coat)
Venom and Carnage
I always liked Nightmask.
It’s kind of funny, I was just thinking last night that if anyone ever started a thread for coolest super-hero costume I’d have to nominate The Spirit (he’s the one at the top of the picture). Okay, he’s barely a super-hero (and even that’s arguable), but what other hero would be able to fight evil during the day and be admitted to a four star restaurant that night, all while wearing the same outfit?
Let me also mention Captain America’s. While a bit cheesy normally, it’s been very effective at various points in history (WWII, after 9-11, etc.).
If you’re gonna mention the Marvel Family, then I have to throw in Black Adam as the best. Bad-ass lightning bolt on black. Nothing else is needed or wanted. It’s perfect because it sums up his powers AND says “I’m a villain” all in one. (As long as they make him a REAL bad guy again and drop all of this “I had a family and they died” crap. Let a bad guy just BE BAD! Sorry, personal annoyance.)
The Ultimate Thor. They made those circles on his chest actually look cool.
Jack Hawksmoor of the Authority. Casual, yet stylish.
Firestorm always looked pretty good.