Any gun I desire, and money is no object; well then I am gUnna go big, really BIG and go for the whole works and say I would buy a Battleship with all guns attached. That includes those guns that shoot rounds the weight of a small car. Yeah, thats the ticket.
Now back to reality for a moment. I would have to say I would really like a Springfield M6 - yeah I know low expectations but it is a neat little survival combo gun and one that could be loads of fun.
Of course I'd also take a mint condition Thompson Sub-gun, or better yet a Luger in .45ACP and I do mean one of the original prototypes from back in the day, not the newly made copies. That is one heck of a pistol.
If I was prone to getting an expensive rifle: how about the Carcano that shot JFK! If it was only one that I could buy with all that money at my disposal, I would take that one!