Most powerful: .30-06 Springfield
Loudest: .44 Magnum (full house loads) ... might not be louder than the above at the muzzle, but it seems louder to the shooter...
And then there's this whimsical answer (which I don't own) -
PO Ackley's Eargesplittenloudenboomer
Apparently he designed this .224 caliber round to break 5000 fps. Only hit 4600 fps, but it is still an interesting wildcat. And probably loud.