Well since you didn't specify, but dangerous begins at grizzly/brown bear, and continues to africa.....starting there, I agree as above, .338 to start, I own a .340 Wby, which they say in alaska, the .338 that is, is the .30-06 to a guy in wisconsin, or the .375h&h if you wer ein africa. personally I can think of no finer a cartridge than the .375H&H to hunt Brown bear, and that would also double as a great african cartridge, can do it all, though on the really big stuff, I'd want more gun. The .458 Lott can be loaded down to .45-70 specs, can make a great all around. My suggestions, .338-.458Lott