If that was the limit for me, and I could only buy 1, I'd buy a Winchester M70 in 30-06.
If I can make my own rifle it would be the same action or a 98 Mauser, also in 30-06.
The 30-06 is now 112 years old, but I do not believe there has been another cartridge invented yet that covers as many applications as well as the grand old 30-06.
Others are a bit better in certain specialties but if you want "one gun" you should avoid a specialty caliber and get a general purpose caliber. That mission statement my friend, is best covered with a 30-06.
It will do everything a rifle needs to do, with about 75% of the field covered with excellence and 25% covered antiquity.
