Ruger Vaquero .357 Magnum.
1. Versatile.
2. Can be loaded to an extremely wide range of power levels.
3. Rugged and simple single action mechanism. Field expedient repairs with basic tools. Parts swappable with other 1851 and 1873 pattern guns. (Even more so if you got an Uberti/Pietta .357. Some Ruger parts are slightly different such as coil springs and separate spring-loaded cylinder ratchets, but other SAA components can be modified to fit in the Ruger frame.)
4. Black powder and smokeless powder.
5. Cast bullets from pocket size molds and equipment.
6. Concealable, yet long range capable up to 200 yards.
7. Big game and small game.
8. Reloading equipment can all fit in an eyeglasses case.
9. Can be drilled and tapped for optics if needed.
Conclusion: If limited to ONE gun only, .357 Magnum master race it is.