Yep "1 world, 1 gun" without question my big game pick would be a .375HittemHard.

BUT then again, if you can jet around the globe chasing big game I think you have the means for more than one rifle!!!
RA

ps
If all I could have was one gun I would want it to be a simple single shot 12ga. That would easily take my head cleanly off with a single round!!!