Wasn't the Old Walker .44 the 'Most Powerful...' until at least teh 357 came out, and maybe until the .44?
And I think the nod goes to Production, there's always some wildcat or 50BMG freak thing that can claim top prize.
So, not to hijack, but is the 500 S&W Mag now the MPHitW (Most Powerful Hangun in the World)?