I've always found this type of question hard to answer.
I tend to find Kira to be the most attractive, but that is partly a personality thing, and I find the character the most engaging.
On pure looks, Terry Farrell is actually pretty spectacular. But so is Nicole DeBoer, really.
In passing, I think that, as a group, DS9 had a really attractive female cast. Penny Johnson (I think that is right?), who played Kasidy Yates, is also really attractive.