.45 Long Colt
It was basically the premier big bore centerfire revolver cartridge. Sure, the cartridge has evolved somewhat over the years to strengthen the case, insure reliability in rifles, and even generate more power in the Ruger Only loads, but even in its original Black Powder configuration it was a powerful round which was designed with cavalry in mind, thus making it effective against horses as well as humans.
The only pity about the round is how much commercial factory loads have been watered down out of concern for old Black Powder frames, so now you've basically got 3 power factors going from pipsqueak .45LC Cowboy Loads which are actually closer to .38 Special than the actual horseslayers that were around back in the day, then there are the defensive loads which are loaded hotter, offering performance similar to .44 Special, but still fall short of the classic BP loads in terms of energy, then there are the hot "Ruger Only" or so-called "+P" loads which rival .44 Magnum. Only specialty loads replicate the performance of the classic BP loads, which launch a 255gr bullet at 1000fps and deliver an impressive yet not excessive amount of muzzle energy, especially out of a 6" Barrel.
Someday I would love to get myself a proper Single Action Army style Revolver, just a fun classic .45 to take to the range.