I think I hit .357 Sig (which I also like) instead of .357 Mag!
I have a lot of favorites, but if only one pistol/revolver allowed, I would take the .357 Magnum.
Depending on the load, good for plinking, small game and self defence.
The .45 ACP would be a close second.
* I reserve the right, to change my mind, in the next poll, on this subject.
** One of each (at least) is the option I like best.