Here's a cop-out answer-
My favorite is the one I have in my hand. It all depends on the mode I'm in.
For plinking I generally go with something in 22rf. For blasting it's usually .308 or 7.62x39 caliber or occasionally 9mm. A lot of times I just want to shoot something "weird" or not common.

It's nice to have the choices that 30 years of lotsa overtime and modest living have got me.