Fabrics: I prefer cotton, though silk or sheer microfiber are nice for a change.
Colour: It has to go with the girl, but classics black, white & red are safe bets, and I personally like dark purples too.
Style: Simple and dressed down for the most part. Low to medium cut. Not too skimpy, I like having something to unwrap, otherwise no point of wearing it. I don't care for kitschy lingire or fetish stuff, it should be like the underwear she'd normally wear, so that seeing and playing with it seems spontaneous & forbiden.