Aside from the obvious ones (personally, I’m a butt guy), I have to go with hair. A good hairstyle can really add to a woman’s appearance, but I really love the feel of a woman’s hair as I run my fingers through it, and I find the slight tickle as it brushes against my skin incredibly sensuous and erotic. It really is something you only get to feel when with a woman intimately, either in the midst of passion or lying on the couch with her in your arms. It’s not something you get to feel with just anyone.
I also think a woman’s voice is extremely sexy.