Boring pick.... BUT.... for me it's the .30-06.... Lots of load options. Plenty of umph for everything in the lower 48. Plenty of range. Plenty of available ammo...
Can load light, can load heavy... or can pick a quality bullet in between and have one load for everything...
Lots of good answers to this question. But my safe will always have at least one '06 ready to hunt with.
-Jake