Anything that can be had cheap, as in cheap parts, cheap insurance, lots of used parts, or something given to you for free. Anything that is old and actually nice would just fuck you if something should ever happen, insuring it wouldn't be nearly as easy, and you might cry if it was damaged.
That is why Mustangs, Camaros, GM G-bodies, any American truck, Miatas, and most things made by the 100s of thousands get my vote. So long as it isn't an american throw away car like a Cavalier or Escort, just pick something you'll like driving, because beboping around in an old pick-up truck isn't for everybody.