I agree the .30-06 is probably the best 1 gun all around, but when talking multiple guns I'd take a .270 over the .30-06 and have other bigger rifles for the bigger stuff.

I think the .270 would do it all decently, superbly on some, decently on others, not for the big bears though, personally I wouldn't have less than a .338 on the big bears.

If you're going for deer etc...for now, I'd say get the .270 over the .30-06 and later on figure out which magnum you want....