A wacom intuos 3 or 4; I'd go for the medium or small sized tablet--the large ones take up too much desk space.
If budget is not an option you could do what I did, I have a small intuos 3 along with a wacom cintiq 21ux. It's the best of both worlds, a small tablet that I can use a pen or a mouse on, and I can also draw directly on the screen of the cintiq when I want to.