Only one rifle? Saying and thinking that is nearly blasphemous!
But if I had to have a single rifle for everything, I'd go with the 300WSM. I'd choose the .30 cal round because anything smaller would have a shorter barrel life. Also, heavier bullets in the .30 would work well on shorter range type hunting (bears etc.).
The 300WSM is an incredibly accurate and efficient round.
I would never have a brake on my only rifle, because that would force you to always hunt with hearing protection. The 300WSM is at the limit that most feel comfortable with for extended un-braked shooting.
I'd have it built to end up around 10 pounds with scope.
As far as scope, on the lightweight end, I'd use a Leupold 4.5x14x40 with Duplex reticle or Nightforce 2.5x10x32 with NP-R2 reticle, on the heavier end I'd select a Nightforce 3.5x15x50 with a NP-R1 reticle.
I wouldn't be happy with a single rifle, but I wouldn't feel undergunned on deer/sheep size game out to 800-1000yds and I wouldn't feel undergunned (very much) if I had to shoot a Brown bear at 100yds either. If I had to shoot a Brown bear at 20yds, I'd feel undergunned though