My favorite? A flat, wooden spatula. Next favorite? Long, wooden spoon. Why? I think because kitchen implements carry a certain feminine connotation to them, and when I'm dominating my sub I like (and so does he) to emphasize my absolute femaleness over him...grrrr...
And I like wood because it has some give and has less chance of injury than metal. And plastic just doesn't slap as nice.
I do also love to use my bare hands to feel his hot skin myself; I save that for when he's all sensitive from being whacked by something else first.
And just because I like putting in my two cents where it wasn't asked for, I wouldn't consider a belt to be spanking; to me that would just be whipping on the ass. Also a nice activity.