Remington 1858, personally. Having an option to quick-swap cylinders in the middle of an engagement automatically makes it more practical as a sidearm than any other revolver here.

If we’re only allowed one cylinder, probably the Colt Walker/Dragoon, probably with a pistol stock to help with aiming and/or recoil. It was the most powerful commercial handgun until the advent of .357 Magnum after all, which wasn’t introduced until well after everyone gave up on black powder in general.

An honorable mention goes to the Walch Navy Revolver, mostly because with how it works, it’s basically a Civil War-era 93 Raffica, featuring a staggered trigger that (to my understanding, correct me if I’m wrong) could mean two shots firing per simultaneous pull of both triggers, meaning if one shot missed, the other wouldn’t, or even two projectiles hitting at once, upping the chances of lethality.