If “everything” means hunting any animal on earth, then what would we use for whales?

If we restrict to land animals, then one needs to search for the minimum caliber allowed for larger animals. I think some African countries require at least the .375 Magnum. My dad used his for rabbits too.

If we constrain to the North American continent, then a 30-06 / .308 are good bear medicine and get used on the occasional mouse.

The lower 48 is 6.5 country.

The .243 is all-purpose medicine south of the Mason-Dixon line.

...and the deer in Texas tend to like the .223.

The guy who said “12 Gage”;is closest to covering all land animals, especially if it is equipped with an adjustable choke that accommodates launching slugs and sabots.