Just one? Hard to say. If I can have unlimited ammunition, too, I'd say either a British EM-2 or a FAL prototype chambered for 280 British.
For a shotgun, I'd love to try running some rounds through one of the CAWS combat shotguns HK was working on back in the 80s, though I think that would again call for a magical source of ammunition, too, since I think it used some sort of proprietary round.
Not sure on pistols. Most any pistol I can think that I'd actually want is something that I can currently buy if I really want it (at least in theory -- better half may frown on a new Sig P210 or something following me home). The rifles and shotgun above are weapons I'm unlikely to ever get to handle, much less shoot.