Music is an art. It's also subjective. But you can compare musicians to one another in terms of music knowledge(theory) and technique.
Versatility, technique, speed, theory, complexity etc. are all ways to measure or compare musicians. However, the creative element is one that cannot be taught, IMO. Guy's like Hendrix, Gilmour, Clapton are GREAT musicians. They are great song writers and are very creative.
When I played drums for a few years and took lessons, my teacher said many of the studio drummers he knew that had PHD's in music theory could play anything. They could take the most complex peice of music, break it down and play it with ease and even improvise on it. However, these guys have come up with nothing on their own and usually write boring music...subjective again, but the point is, technicality is only one measure or quality of a musician.
Guy's like John Petrucci, Satriani, Vai, Michael Angelo, Yngwie are virtuoso's and masters of the instrument. Does it mean they are better then Tom Petty? In some aspects yes, some no!
Frank Zappa had many drummers audition for his band. The music was very complex and had many odd time signatures. Only Vinnie Colaiuta pulled off the audtion against other drummers and became his drummer for a few years. He then went on to make Joe's Garage/Shut up and play Yer Guitar which are two modern classics that raised the bar in drumming. John Bonham, of Led Zepplin, who was a drumming legend, wouldn't have been able to play the peices Zappa wrote, simply bc they are too complex.