A Coop-de-Ville?

Actually, whatever he decides has the best safety rating after performing extensive research. Perhaps a tank?

This:

http://pics.imcdb.org/5217/brady34h3919.jpg

A Maxi-Cooper to correspond with Sheldon maximum brain capacity. I remember that Sheldon said that he did learn how to drive and got his drivers license, but chooses not to drive anyways. Its so much more convenient to have Leonard drive him around.

It would be a Maxi-Cooper because of his ego.

Even before he got his license, he said that he CAN drive, but chooses not to. He tells other people - and himself too, it seems - that it's because he prefers to feel special by being driven around by others. But the reality is that both in testing/simulation/driving games and in actual driving he... didn't do well.

I expect the driving test for his license didn't include freeway driving, or Sheldon's lead foot would have guaranteed failure.