Tough choice as I have owned and built so many. I can't identify a perfect mountain cartridge as the blend of the cartridge choice, bullet selection, and overall rifle design blend together like an artistic preference. My original "go-to" was a Finnlight in 270WSM, then a 30-06 700Titanium, then a 280Rem built on the previous Titanium, all replaced by another Remington Titanium in 260Rem. Tried a 308 and a 7-08, loved both, and could easily argue the case for choosing only the 308. Now a 270Win and a 35Whelen.
Out of all of these rifles, I would narrow down to the 270WSM and the 35Whelen. The 270WSM can reach out and hit hard, short action, lighter rifle, low recoil, and plenty of impact. My 35Whelen does not have quite the same reach as the 270WSM but it is a very light rifle and where I go back to my previous comment on the blend of components and the need to be tailored to individual preferences. Overall a light build, easy to carry, very accurate, and can reach out to 400 metres with 225 and 250 grain loads. I take the 270WSM when I go above the treeline to chase sheep as I expect the longer shot across a canyon and the 35Whelen when I am in the trees and hiking ridgelines for elk or bear. After all that being said, I would sell my 270WSM and only ever carry the 35Whelen for all my hunting.