I was in V's Secret a few weeks back and saw a navy blue silk camisole...just the right details to make it sexy in a classy way, and a little bit of lace but not enough to make it scratchy. There were matching panties, as well. Can't remember if it was a bikini or a thong, but either way...
I loved the camisole, but couldn't think of a good enough justification for buying it.
But I guess that sort of thing is what I'd wear right now, were I wearing lingerie...something silky, soft, comfortable, and yet streamlined and sexy. Nothing long...things that hide my waist make me look like an inverted triangle, so if your girlfriend is pear-shaped and doesn't have very large breasts, you might want to take that into account. A waist-length (or slightly below-the-waist) cami is probably a better choice than a negligee.
When I first started wearing lingerie in my late teens and early twenties, I was, of course, fascinated by teddies, garter belts, stockings, and crotchless panties...probably because I hadn't ever worn them before.
They got old, though (as did red and black...I agree with whomever nixed that combination earlier).
Now I like classy colors like white, navy, dark burgundy, dark green, or black.
Probably the best advice when buying lingerie for a woman is to pick something that will actually be comfortable for her to wear, as well as flattering to her figure. I'm sure the women working in any reputable lingerie shop will be able to help you figure this out. Work up the nerve to ask for their help -- you won't regret it.