As much as I prefer the .45 ACP, if limited to one caliber for the rest of my life it would be the 9mm.
Why?
The 9mm is easier (and less expensive) to get worldwide.
Our own military, as well as the NATO countries use it, insuring a steady flow of cheap, surplus ammo well into the future.
In the same weight / size envelope, you can fit many more 9mm rounds.