Honestly, don't know
If I had to guess, 243 Winchester. All the attributes of the 6.5 Creedmoor in its day, low recoil, great velocity, accurate, good caliber for kids or recoil sensitive shooters. With a soft pointed bullet, killed stuff dead at reasonable ranges. Too bad Winchester hamstrung it with the slow twists. Imagine what it would have been with an 8 twist or even 9 twist barrels and 68ish years of technology advancements along the way.
I know my uncle had a 59 Model 70 in 243 and it killed everything he pointed it at my entire life, elk, deer, antelope, gophers.