7mm, because it encompasses the 7mm-08, the 7x57, 7x61, 7x64, 280 Remington, 7mm Rem Mag, 7mm STW, etc.
The others only cover one catridge, why have one when you can do everything? (Sure that's not the way you meant to write the poll, but that's the way it came out).
7x57 would be my choice for a one-size fits all rifle. After that, .30-06, .308, and .270. 35 Whelen would also be a solid choice.