Butt. And that's not just about size. Ideally it's a combination of both size and shape, but shape is more important to me.
Last girl I was with had a small butt, but it was athletic and shapely. She had a flat chest and was self-conscious about it, going so far as to hide them, even though I couldn't care less.
I will say, I have gone out with a girl that had double d's and that was great too. But I'm an ass-man for sure.