As I understand the question, it's not "what gun would you choose if you could only have one gun?", it's "what gun would you add to your collection if you could have anything you want?".
If that's the question, I'm with thvigil11, give me a perfect, complete Singer 1911. If it's a gun to use rather than just have for a collection, give me the BFG-9000
Oh wait, you meant a REAL gun? Sorry, can't pick just one ...