It is a tough one but I'd have to say 22LR. Cheap, accurate, and commonly found. It doesn't do much for hunting anything bigger than about 12 pounds and isn't the best choice for self defense but it is something that is not intimidating to the shooter and one can shoot it alot/often with little cost.
In centerfires I would pick the .357 S&W magmun. It is a little light for big game in my opinion but if used in a large N-frame gun rather than the mid-size K-frame types, one can get the max out of the cartridge. One could also fire the much cheaper 38 Specials for practice or even small game and either would work for defensive purposes. It is the cheapest "all purpose" cartridge out there in terms of ammo and gun cost.
Well looking back over the the years I'v fired most all
popular calibers from 22 short to 44 mag. My favorite would
have to be which ever one I happen to be carrying at the time .Noise wise the loudest was the 222 in a 10 inch
If I had to pick one it would have to be the 357 magnum. I
know it will kill a deer because I did it.
Like a lot of you, I have handguns in calibers from .22 through .45 and a number of Thompson barrels in rifle calibers, but my favorite would have to be the .22lr. That's the one I am most often walking around with.