OK, then, for those that need to read the OP again, and still argue, here's why the .30-06 is the leader here. The OP uses two different defining words: 'ultimate' and 'best'. He then defines that further as being capable of taking anything, not just limiting the hunter to NA big game. When I said the the .30-06 was the bare minimum for Africa, that was to accentuate the point that if it's the minimum that guides will allow there, it is sufficient, more than sufficient, for the biggest on our continent.
So, for clarification, the .30-06 can utilize bullet weights from 110gr to 200+.... no other caliber can boast of that range. I've loaded ammo for mine from 110 to 220 grain, and it is able to handle everything I've thrown at it, obviously with results varying from one bullet weight to another, but still capable of 'hunting accuracy' with nearly everything.
Ammo availability: no other caliber with the exception of the .22LR is as prolific. I dare anyone here to hit a random business that sells ammo that can't find something for you in .30-06. Can you say the same for the .338? .35 Whelen? Didn't think so. Granted, the .300 Win Mag and .270 come close, but still can't catch it.
By those criteria, the .30-06 is the 'best' for a one-caliber shooter as it will take anything from squirrel to Grizzly, ammo can be had anywhere, the recoil won't kill you, etc., etc.