Versatility.... or popularity by number sold?
In reality, probably the latter.... people make do with what they' ve got and doggedly tend to promote what they' ve got.

Assuming we are ruling out the 22LR (most sold), then....
North America: probably the 30-06
World Wide: probably the 7x57

Most versatile, eh?
A dubious honor don' t you think?
Jack of all trades and master of none?
(Like an 8" crescent wrench being deemed the most versatile tool in the box, but don' t try ignition work with it and don' t try to change truck tires with it.)

EKM

With the different bullet choices and weights the 30-06 has to be right on top.
Ruger Redhawk

By versitile I ' m assuming we' re talking about the different loadings from the factory has to be the .30-06 You can get 55 grain bullets for varmits and you can get 220 grain for moose, elk , bears and it has choices in between.

That' s like asking what' s the most versatile shoe- can it be used for running, slogging through a swamp, worn on the construction site, and then polished off to wear to church on Sunday?

I would say the .308 or 30-06, though neither is the best for everything, and might be completely inappropriate for some purposes.

30-06 gets my vote

.30-06

55- 220 grains availalbe. Can be used on anything from woodchuck right up to the biggest North American game. (Not necessarily the best choice for a few animals, but it' ll work)

Shells can be found at almost any little shop around as well as every larger chain type store that carries hunting equipment.

Good article on this subject:

[link]http://www.chuckhawks.com/all_around_cartridges.htm[/link]

I assume you are limiting your hunting to our continent. Since there are some very dangerous critters on our continent, and I have hunted and killed griz, the most versatile round is one that is adequate for the most dangerous game. Guides will tell you that something in the .338 class is good. Also, it does a good job on prairie dogs (plenty of penetration).

Just out of curiousity, those of you touting the 30-06 with the 55 grain bullets, have any of you actually shot accelerators and if so did you hit the broad side of a barn with them? I have never seen any rifle chambered in 06 that would accuarately shoot anything under 150 grains so I am just curious.

frizzellr, I haven' t , but like I said i took the question to mean what caliber has the most different loads.