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I'm not sure anyone mentioned so far qualifies as a terribly good shout, or that anyone on the planet has sufficient knowledge to answer the question. These days with injuries being more commonplace and eight subs making it on to the field, anyone who's in the top three or four in their position for their country for any length of time is going to get a cap at some point. It seems far more likely that the greatest ever to never win a cap comes from an era of consistent selection policy when you had to take the field to win a cap and in England at least, play for a fashionable club. Not saying he's the best shout, but one such player is Penryn's Roger Harris, who sat on the bench behind John Pullin some 20 or 30 times but never won a cap. Before the league structure in England, I don't doubt that there were many other players who never received the recognition they deserved because they were playing for their local clubs in a county that didn't fare well in the County Championship.