Its pretty easy.
You just gather every guitarplayer in the world and measure their knowledge in every technique
available, all the scales, their speed, their ability
to make their own music, etc etc etc etc etc.
Then you have to figure out whats most important;
for example in classical music speed and scales are often important. After you've ranked all those different
things in every style of music in the world, you can
start creating a scoring-system.
Then you must look at all the guitarists different styles, how original they are, how "correct" they are (visit your local music-university for help) and so on.
Then there are about 1326 additional things you have to consider, and then you can start measuring.
OK its hard, but its not impossible.
It will probably cost money to be able to find out how to bring some of the guitarists back to life, but you can probably get some sponsors.
Tell me when you have got a winner, I want to know
who it is.
Good luck!