"Everything" huh?
That'd throw out everything but .22LR & the .30 cal stuff.
Much as I love my .243 & .22-250 (the .223 just isn't "there" considering the rest in the ball-park), all the "twixt-stuf"f is covered by any .30 cal & no need if going "down" to the LR.
& really, if you've a .243, what're ya giving up for a .25-06? Same-same for a .270
Be froggy if you like, I've calibers that span almost everything mentioned, but for a "one gun" thread? Only difference is more personal preference & a shade here 'n there regards trajectory - every one will do the job - some a hair "better," - if that.
Don't need a magnum as the other two .30s 'll do it fine enough.
-06 has a bit of an edge, but doesn't get me anything that a .308 really wouldn't - & zip for surplus these days in comparison.
Recreation wouldn't figure any longer as I'd feel we'd be in "the worst of times." Same for target shooting. Ditto varmits.
I'd have to have something to survive with.
(Should have asked that question to begin with.
)
I'd hate to part with any of my .308s, but would have to go with a pretty bare-bones .22LR - the quieter the better.
Hunting skills - that of sneaking up, or staying away from - would be paramount. & likely any shots would be few & far between.
Did I mention quieter the better?
Choose whatever makes you feel fuzzy. I'd just as soon stay alive.
Actually, a firearm wouldn't figure much in
that mix after all.