Men: Back. Definitely. I love back muscles. Or voice. I find myself falling in love with a lot of guys who just sound like a dream when they talk or sing.
Women: Back as well, and collar bones. Oh, and I absolutely go crazy for beautiful eyes.
Yep, feet would come at the top of the list for me too, I'm also into legs (from just above the knee down) necks, hands and shoulders, I think it's to do with the way these areas are more defined than the rest of the body.