I’ll cast my vote for the 303 British!
Probably the most prolific sporting (and military) round in Africa, Australia, India and North America as far as single shot rifles go.

As far as the greatest dangerous game single shot caliber, generally, it would have to be the 450/400s or the 450 3-1/4” imo.

But we all know the top of the heap is the 461 Gibbs No.2! ;-)