I still have my first 30-06, a 1963 model Winchester 70 featherweight, my father got for me the year I finished high school. Later I picked up another, an early 80's push feed model and like both. My other two are an M-1 and a Ruger #1 standard. Other than AR types, the only caliber I have 4 rifles in is 45-70.
If I could have only one centerfire caliber it would probably be 30-06. My second choice would be a 375 H&H, generally throwing bullets 90-100 gr heavier than the '06 at about the same velocities and trajectories, a bit heavy to shoot varmites with as compared to the '06 but better for heavier and more dangerous game.