Hu, quality has nothing to do with rivalry - sheer hatred is what determines football rivalries, as far as I'm concerned.
This isn't your average Man Utd - Liverpool or Paris - Marseille type derby. We're talking about two sets of fans who not only hate one other because they're the two biggest clubs in the country and therefore are already rivals, but because one set of fans are catholic and the other protestant, one set of fans believe that Ireland should be united and the other that NI should remain a part of the union.
Celtic and Rangers is a war between Ireland and Britain on and off a football pitch. It's without a shadow of a doubt, the most bitter rivalry in the world, no other rivalry comes close, none.