I keep hearing that the .45ACP is typically the most accurate, at least by a little. I also like that there is more lead wadcutter ammo available for it than for other auto calibers.
Old timers seem to be largely of the opinion that, in general, the faster the cartridge, the less accurate it is, at least as far as handguns go. Horace Kephart believed that sectional density had to increase with velocity in order for accuracy to be achieved. This would jive with the aforementioned opinion about handgun accuracy, as well as with the fact that high-velocity rifle rounds such as the .308 can be very accurate. I still have alot to learn about ballistics.
I just like making tiny groups on paper. whether it's needed for practical purposes or not. Call it a fetish. All of my guns end up squirrel hunting too, whether I planned to or not. I'm, just hardcore like that.