Most useful in a variety of situations or on a variety of game? I could probably shoot everything I hunt from now on using nothing but the .308, but using it on some critters, both large and small, would properly be categorized as "stunt" shooting. For North America, I think that the .243 Winchester is probably truly more useful for a somewhat greater variety of game than the .308.
That said, the .223 is probably regularly used to take a greater variety of game these days than any other "short-action" cartridge, although it wouldn't be my first choice.
