Easy, .22LR. It's the caliber I have the most fun with. Blasting rapid-fire for practically nothing, plinking cans or even bottle caps, targets of opportunity when tramping through the woods. Even settling down to a leisurely day of 100 yard bench-rest for literally pennies. Nothing more relaxing.
If centerfire is a must, then it is .45 ACP. Simply put, it is the caliber I shoot best, 22LR notwithstanding. In one of it's best defensive loads, it is contained in my EDC. It is also a pleasure to reload, as my pet load's powder charge is about half-way up the case and is therefore easy to spot as the case comes around station 3, and though my Dillon's powder bar has never failed, a double charge would be a no-brainer to spot. I too am a fan of the 9mm as well as the 10mm, but .45 ACP will always be my favorite centerfire.