Yeah, Catwoman's up at the top of a lot of lists, but for me, Harley Quinn, all the way! she does things for her costume that no other toon ever has! And the way she wastes herself on that unappreciative slob, the Joker! It makes me boil.
Others:
Vixen (Mari McCabe)
Fire (Beatriz de la Costa)
Dawnstar (LSH-- gotta love those wings)
Sue Dibney (hey, if she can keep Elongated Man happy...! I hope they bring her back! Ralph deserves a little hapiness in life.)
Plastique (fought Firestorm and Cap't Atom)
I forget her name, but she was the PR liason of the JLE
Dream Girl (LSH-- what else can you say about a heroine whose powers only work in bed?)
Alanna Strange (Adam Strange's wife, on Rann)
Abby Cable (Swamp Thing's girlfriend.)
Lana Lang
Lyla Lerrol (Superman's great love, on an early 'Return to Krypton' story, in the sixties.)
Batgirl-- BOTH Barbara Gordon and the new one who wears the full-leather outfit.
There are others, but that'll do for now. Are you familiar with FemForce...? You might want to check them out. There is a cosplay fantasy site about them, somewhere. They are, to put it mildly, dreamboats.