One handgun for everything/anything? That's asking a lot of one gun. Probably a S&W Mountain Gun (629) in 44 mag. Gary Sitton made his case for it years ago in an old Guns and Ammo article entitled "The Last Gun", makes sense to me even though I don't own one, I'd have to make do with my 4" Redhawk in 44 mag. My handloads could make it suffice for anything I would ask of it, 240/250 grain H.P.'s for stuff they're good for, 330 grain LBT's for the big, nasty stuff with teeth and claws, and maybe 180 grain cast for plinking/small game.

I'll also confess an interest in some kind of auto pistol in 10MM, never owned one but I might make that work also.

Deus Juvat