It all depends on what kind of sound your going for, how much your wanting to spend, and if you want 1 or 2 subs. I like 12" subs primarily because they hit a decent range and fit in pretty much any car.
If your going with one sub and you want it to push a lot, sound pretty good, and not cost very much i would go with the Pioneer Premier TS-W3002D2. you can find it after shipping for about $160 and i would recommend building the box.