Not so much a costume but a statement of elegance in design. It's so good in that the only way one could improve it would be to create a tranny version. (pause)
I just said that, didn't it?
OK, when I think of The Flash, I still revert back to my childhood. My first encounter with him was in comics, where he was a sort of bland guy who fought guys who seemed to really just want his undivided attention. In a world where I could go from an Adam West Batman to Neal Adams' Batman, The Flash always just sort of shook me. For one thing, The Flash's power was to be "the fastest man alive" but Superman always beat him or would get so distracted that he couldn't finish or he'd simply be a jerk and just throw the race, winking at you at the end of the story like a fourth wall breaking d**k we all knew he could be. When there's a guy named Superman who can do everything everyone else can do, well, it's hard to stand out.
So, no, The Flash wasn't always a favorite.
And then, Wally West became The Flash and suddenly artists seemed to find something to work with. Speed lines weren't gonna cut it anymore the costume seemed to almost come alive. Artists played up his lightning motif and did the impossible; finding ways to make "The Fastest Man Alive" look good even standing still.
It's a red leotard and leather boots and it just works. Donna Troy's costume like a Hilary Swank is so extraordinarily severe in its visage, it borders on the sexy.
From the color scheme, to the scalloped trunks to the pixie boots this has ZERO reason to work... and it really doesn't. What does work is that as a costume, designers have consistently found ways to build upon what was already there. Yes, leggings have been added, boots, as well but overall, no matter who wears it, the costume has essentially stayed the same.
What I always thought was the sheer brilliance of the robin costume was its juxtapostion with The Batman's. You instantly underestimate whoever's wearing it... that is until you're... you know... waking up from being knocked the hell out.
Look at it.
LOOK AT IT!!!!!
It's so sleek. Not a bit of wasted design. It's simple. It's green. It has a lantern symbol on it. From the white gloves, giving it an almost military dress feel to the perfect placement of black, it's in my opinion, the cleanest looking costume design ever created. And let's be honest, it's a sexy piece of design work.
The other reason it's so great is that even though it's near perfect, it can be varied and made to fit the personality of the wearer. From the battle-ready gear worn by Guy Gardner, to the goofy, unkempt look that's come to be associated with G'Nort or the overt sensuality put into Arisia's look, no matter what you do to it, it always manages to say; Green Lantern.
It says who he is before you have a chance to ask. From the bat symbol in the middle to everything north and south of it, The Batman's costume is the measure by which I hold all superhero costume. Every element has a reason, every element has a design function. Bill Finger created a design that over 70 years later is the costume everyone has been trying to re-design. Not so much because there's anything inherently wrong with it but more out of a silly belief that perfection can be improved upon.