My favorite, the .45acp. But if reloading is an option, I'd probably go with the 44 mag. I can load it up for deer or down for bunnies. And my Ruger SRH is pretty accurate and both ends of the scale.
I like .40 S&W best in automatics. I find it a good balance between capacity/size and stopping power. I also enjoy the .45 ACP. It is a proven, powerful round that I shoot well with. I will accept smaller calibers than .4 diameter, but only in carry guns, not in compact or full size.
In revolvers, I like the .357 Magnum. Variable loads from .38 Special strength to serious stopping power.
After shooting the popular calibers and owning 9mm,.40 and .45, I've found the one and only caliber that I shoot the most accurate. The .45GAP is my favorite, after shooting it I sold my 9mm and .40, and won't buy another Glock unless its chambered in the GAP, I like the 200 grain Gold Dots out of my Glock 39. The GAP gives me a .45 in a 9mm frame, standard 6+1.
Knee-jerk answer, based on the simple subject line and focusing strictly on emotion and my personal history with it: My round is the 10mm Auto.
In reading more on the questions posed in the original post, my next knee-jerk reaction is .357 Magnum, because I'm a handloader and I can easily make it run from mousefart to hellcannon if I need or want to.
But if it's a simple "as it comes across the counter" ammo, and not ammo that I can make very light, ultra heavy or anywhere in between but rather, only as it comes from a factory box, maybe my best answer is .45 Auto. .45 is not too heavy or ridiculous for a lot of shooting, high round counts and long days with many, many rounds of ammo launched. It's also quite capable if put in to defense duty.
My round? They should put "10mm guy" on my headstone and they can walk by, read it, and wonder about it... come to their own conclusions.