As Les Paul said, you can never say someone is the greatest, because they all have different styles and different ways their great. Some are great technical players, hitting every note with precision, while others are great impromptu players making it up as they go along.
Eddie VanHalen is one of the best rock guitarist in the world, but when he went head to head with an an old country boy named Chet Atkins, he found out that he couldn't keep up with a Bluegrass man.
And to be truthful, when someone like Hendrix is lost so tragically, we all remember them as being better than they really were, not that he wasn't one of the greats. And he didn't play upside down, he was left handed, and because they didn't make left handed Telecasters, he had a right handed one strung upside down to make it left handed...