Um... wow.
I can totally get behind Hal Jordan (and indeed most of the variations it has spawned, including Guy Gardner's punk/biker variant and John Stewart's current look, based on the Bruce Timm animated design) and the Atom. And Spidey is a straight-up classic.
Some of the other possibilities mooted here... well, least said, soonest mended.
I quite like the modern version of the Nightwing costume--very simple, very sleek, and a refreshing change from 50 years of bad costumes for Dick Grayson (with the last version of the Earth-2 Robin costume, designed by Neal Adams, being the only other serious contender).
I think the basic Barry Allen Flash design, and its variants over the years, are pretty awesome. That design has come to all but embody the concept of speed to me.
Jumping ahead to the 30th century, I have always really dug Mon-El's classic costume. There have been variations on that theme (the 90s' Valor series, the post-Zero Hour "M'Onel" costume) that never quite worked for me. But the essential military-style red tunic and pants, with blue boots and cape fastened by big gold buttons, just really clicked for me. I also liked Keith Giffen's slight modifications to that classic design toward the end of the Vol.3 Levitz/Giffen run... he gave it some futuristic details without really changing the essence of the design.
(Come to that, I thought all of Giffen's redesigns at the end of that run were pretty awesome. They gave a fresh, high-tech look to the Legion that really distinguished them from the common 20th century spandex look, and arguably predicted the practical/cool/'realistic' look of the X-Men films better than a decade before the fact....)
Dave Cockrum's redesign of Lightning Lad's outfit was pretty awesome, too. Most reimaginings of Lightning Lad since then have owed an obvious debt to Cockrum, but his is still the best.
Overall, I really liked the 'uniform' style Chris Sprouse introduced for the Legion in the mid-90s. That style was kept after the Zero Hour reboot with few major changes.
--g