Okay Wrong Way, I'll bring the 7mm in.
If I had to part with all but one of my rifles I'd keep one of my 7mm Remington Magnums. Quite simply, I'll never find myself hunting outside of North America and I cannot honestly say I am as comfortable with any of my other rifles.
There are .284 calibre bullets ranging from 120 to 195 grains. A shooter can cover varmints to large bears.
Hand loading beyond the anemic 7mm Remington Magnum factory loads, and use of premium bullets really bring these guns to an impressive level of performance that the short magnums may not be able to match; in fact, I cannot immagine getting myself into a situation that would lend any real-world advantage to a .300 Mag over a 7 Mag.
Sure, some calibres may be better for some applications, and the short magnums may be more efficient and inherently more accurate, but IMO the broad range of capabilities and applications represented in by the 7mm Remington Magnum is very difficult to match for the hunting that I foresee for myself.