I have a 222, 243, 270, 30-06, 7 mag, and 338 win mag.
If I had to choose only one, it would be the 06.
IMO 270 and down is not enough gun for me to feel comfortable at the top end and the 7 mag and up is more than enough and kicks hard!
06 shells are usually cheaper due to volume sold and are available almost anywhere. They offer 125 gr reduced recoil loads (equivalent to a 243 in power) up to the 180 gr light magnum loads (equivalent to a 300 win mag). What more could you ask for in one gun?