Black Orchid.
Marvel Girl’s Phoenix costume, especially the way it was drawn during the Dark Phoenix Saga.
Or the Poison Ivy suit from NML.
You’ve all made good suggestions, but I still think Phantom Lady deserves the title because the character was being draw in a time before the Comics Code clamped down on overt sexiness, and before the more recent era where comics were being drawn for adults and oversized breasts became the norm.
Hey, seduce me, Phantom Lady! I’m innocent, I swear!
Powergirl, at her, uh, “artistic peak”
Batman’s.
If he’s…oh, forget it.
Xmen is always good for sexy costumes. I always liked the Black Queen and White Queen costumes myself (not technically heroes, I know), particularly if Jim Lee was drawing them.
The link wasn’t working for me though.
Can’t believe no-one’s mentioned Barberella yet! She just has the best costumes - I’ve modelled myself on her ever since the first time I saw that movie! I suppose you could argue she’s not a superheroine, but sod it, she’s my heroine!
mmmMMMmmm…Fonda!
Mind you, Barberella wears about half a dozen costumes in the movie. Did you have any particular one in mind?
I was fond of the outfit she wore to take the call from the Leader (you know, right at the start of the movie!).
Just a note – when researching for examples to link in this thread. I discovered that there is a huge fetish apparently surrounding Super-Women, bondage, and such.
Am I just an innocent 45 year old guy who never got it or is this some kind of common knowledge?
Are you kidding?
There’s not only a big intrest, and many sites, but there’s even an online comic strip lampooning it all.
Look up “Topless Lass” or “Buxom Girl”. 
I’d have to go with:
Phantom Lady (a personal favorite of mine, and very Bettie Page-esque with her jet-black hair and penchant for bondage)
or
Zatanna (tuxedo, top hat, tails, and FISHNET STOCKINGS)
I like Ghost’s mysterious retro-glamour a lot too, and for sheer kinkiness, you can’t beat the White and Black Queens (by Jim Lee, natch).
Any costume that prominently displays the male crotch.
None of y’all’s Yahoo links are working. 
Captain Liberty:D
(You should see it from the back!)
Black Canary.
Bustier. Fishnets. Blonde wig. 'Nuff said.
This thread really needs this link
Or any thread, for that matter.
Seriously, that site is a geek’s dream come true. SUPERGIRL!!! Add to favourites, indeed.