I suppose this could be open to any vehicle, it's just usually a "dream car" for people..
No price range, no size limit, nothing. Anything you want, PICK ONE!
It's easy for me.. The car I had that I loved most (as far as fun to drive) was my 1999 Tin Can Death Trap (Miata), and it was what I consider to be "lightly modded".
Reason being the light weight. If you can't go fast on 90hp, 900 won't help you. (My favorite car quote)
So with that preface, I give you the epitome of nerd-core: The Caterham 7 Superlight R500 (Caterham Cars)