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