Most powerful rifle I have would be my 416 Rigby, it has a slight edge in energy over my 458 Win Mag, plus being faster it gets the edge in penitration when you use solids in both guns.
Also the Rigby has the advantage of range.
The Rigby also has a huge advantage over my 458 or any other rifle I have when it comes to felt recoil on the shoulder.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071