"I really like your sheets."
It's hard to know what people will take to heart and what will go in one ear and out the other, so it's hard to say the best compliment.
I told my girl she rides like a pro, though, cause so few girls actually know how to ride on top. The best compliment I can give, though, is no matter how turned on I am, or where I'm at on my road to orgasm, I almost always come with her as she cums... even if I'm not thrusting, just being in her is enough - it's only her, too, doesn't happen with other girls. Weird, freakish connection. I'm drunk. The end.