Well, the easy answer, regarding caliber, is .357, which is the bore diameter of several interesting and useful cartridges, whether designated as being .38 or .357. From low-recoil plinking, to defense, to quite long-range tasks, the several cartridges, that use .357 bullets, are collectively, much fun, and quite versatile.
My favorite firearm, with nothing else coming anywhere close, is my first GP100, which I used in a deadly-force defensive incident. Meaning, we each had deadly weapons, and, I remained standing, and he did not. So, my GP100 being chambered for .357 Magnum does have some influence, here.
Had the question been “favorite cartridge,” I would have had a more difficult time providing a quick answer. I have much deep and abiding affection for both the .45 ACP and .45 Colt, and much interest in the .45-70 Government. (The 45-70 does have a considerably larger bore diameter than the .45 handgun cartridges,) These cartridges warm the heart of a US history buff. Even so, the .357 Magnum may have won that won that contest, too, had that been the question.