Most powerful in terms of ME is my Ruger #1 Tropical chambered for the .416 Rigby. It is a gun the I have absolutely no use for and which I thoroughly enjoy.
The same could be said for my Winchester M70 in 375 H&H
Loudest? I don't really know.
Hardest kicker? In terms of free recoil, it is the .416. In real, shooting, terms it is a T/C Encore with a fast twist barrel from SSK Industries chambered to shoot this round:
A BMG bullet seated in a 50-70 case. Subsonic, it comes out of the barrel at about 900 fps. Nevertheless, it is the only rifle that has ever caught me with its scope. The recoil pulse is so fast that I could not get my head out of the way. Glad that I had safety glasses on. I had to go to a LER scope.