#1 is a tie; 44mag and 45ACP
2nd probably .357mag
3rd 22lr
Honorable mention .375JDJ
I like larger rounds that make larger holes in a pistol cartridge. The 45acp is pretty good at that. It has so many pistols to fit, and has done so well in competition.
I really like the 44mag. Others have come in more powerful, but the 44mag still has decent power. And, it has proven itself for accuracy. My favorite combination is 44mag in a Desert Eagle. With my old setup, before UPS lost/stole it, I'd shoot 100m steel targets for fun, offhand, unsupported. That 44 had the accuracy and power to knock down the steel easily. And, it fired so smoothly in that Desert Eagle...
I like the combination for the .357mag(and 44mag for that matter) to load easily as hot mags or light specials in the same revolver. I also like the 357 revolver paired to a lever action like the Marlin 94CS. Sweet!
The 22lr, well it is cheap! It's nice to shoot with the 44's too; the big bore stuff will make you flinch eventually... The 22lr is a nice plinker, cheap, and easy to shoot.
The honorable mention... I had the opportunity to run a bunch of rounds thrrough a TC Contender barreled for .375JDJ. If you are not familiar, it is a .444 Marlin round, neck formed to .375. I got some praire dogs with that, at 200+ yards. It was scoped. I was sitting, and resting the pistol on my knees. That pistol and round are noted for exceptional accuracy. The owner has taken deer, antelope, and elk with the setup. Oh, and prairie dogs...