My wife has grown fond of cowgirl. She can control the motion maximizing deep a-spot stimulation and grinding her clitoris on my pubic bone. I've gotten lazy and just let her grind away!
When I haven't joined her for an orgasm that way, we usually go for doggie style. I can grab her hips and get a good pounding rhythm going.
We do some other things like missionary using the Liberator Wedge. The angle created by the Wedge improves g-spot stimulation for her.