This is a question I'll never get sick of contemplating!
Current favorite is the 270 WSM...a 140 grain Berger Classic Hunter at 3300-3400 fps is lightning in a bottle. Bought my 270 WSM specifically as a mountain muley rifle...2 for 2 so far.
For timber WTs in NW Montana and N. Idaho...my .243 gets the nod...5 pound rifle, light and handy for still hunting...enough horsepower to reach out to 4-500 yds in some of those burns/clearcuts if needed, but way more comforrable inside that 300 yd range.
Rifles, shotguns, and fly rods...a man can never have too many.