I am going to assume you mean one big game rifle and one load for the rest of my life. If that was the intent of the question, then I will probably say a .30-06 with a 180 grain partition. I don't have a .30-06 that I presently hunt with, but noone can say it isn't sufficient game caliber. Partitions have earned a reputation that is as reliable as the old "Rare, naught six".

"The number one problem with America is, a whole lot of people need shot, and nobody is shooting them."
-Master Chief Hershel Davis