Excellent, 120! Yes--I must agree with you that, in a competitive environment there is generally a winner and a loser (or several). Combat, politics, business, sports, most if not all games, and so on.
But I didn't mean to imply (and I think I may have) that a 'best' in any field implies a winner (and therefore, a loser). But that's where the need for a 'best' probably comes from, just the same. For example, we may refer to the best car, the best horse, the best athlete, etc., in the context of races--again, situations where we do think of winners (but not so much losers, interestingly).
When it comes to musicianship, however, 'best' is not testable, is it? I mean, it's totally subjective--again, more like other arts. Well--okay--I'll have to take that back!!! It's certainly conceivable you could do this: in the manner we 'test' Olympic figure scaters. A panel of judges who rate in one or more fairly-well defined categories could do this. In the world of performance then, it's the audition.
So there you have it--I recant my rant
There IS a precedent that most folks apparently either accept or at least are accustomed to, and that's all the original poster was trying to emulate, perhaps.
Technique, originality or creativity, versatility etc., might be rating categories that could be 'measured' by defining examples of these things (you know, triple-reverse-axle on three drums, etc).
I think what you'd get if you actually did this experiment is a number of 'tiers' of musicianship--some folks would fall into Tier A and would all be so close in their scores (i.e., 9.89, 9.90, 9.87, 9.85) that it would look like averages for the top 100 PGA players--you can't tell 'em apart. And, if you were to select a different panel of judges or judge on two consecutive performances, the 9.9 guy would get a 9.8 and vice versa.
One more thing and I'll give it up--but a problem I see with doing this kind of rating is that I think you tend to drive toward demonstration of technique or speed or something else that may not necessarily equate to musicianship. We were just having this dialog on the ibreathemusic.com forums. The idea: guitarists who may demonstrate exceptional command of speed and/or music theory can produce music that is more a demonstration of those skills than it is enjoyable music.
So, best guitarist may be good musician, but not a great musician. Kinda subjective, eh?