I assume that the idea here is that your "One Gun Only" would be a handgun. I only say that because in the grand scheme of things, a rifle would be far more useful.
Anyway...
One handgun... Geeze...
Right now, it would have to be a SIG 239 in .40.
First, it fits my hand like no other gun does (although I do plan to try to get som ework done on my 1911 to make that work better for me)
Second, it is as reliable as the sun coming up every day.
Third, it can shoot just about any bullet profile and style I want from lead ball to jacketed hollowpoints.
Fourth, the factory is in NH and a fairly quick ride if I need a part.
Sixth, I already have it.
Seventh, Did i mention its reliable?
Eighth, It hides well on me in various modes and holster types.
Ninth, Cheaper to feed than a .500 Magnum not to mention it's legal for IDPA unlike the big 500.
Tenth, I just bought a case of ammo and two new mags for it for IDPA this year. (^_^)