Definitively the UNESCO 3-level classification is a good one (it covers subjects with detailed coverage with its 2/4/6 digits levels), it's well-known and used and those codes are perfect (standard) to be used everywhere.
My only concern is that I remember some discussions time ago about it lacking some important knowledge areas (it sound to me, for example, the lack of support for Physical Education subjects, for example, not 100% sure tough).
Perhaps we can expand it to use those missing subjects (99xxxx) if necessary... but that will break the official classification.
Also, I wanted to point about the GEM classification system (
http://www.thegateway.org/about/documentation/gem-controlled-vocabularies/vocabulary-subject) perhaps not being so "universal" but, in practice, it has a good (and simpler) coverage of subjects (I didn't get objections from people when I asked about it some time ago).
Negatively, it hasn't proper "codes" for each subject, but well, that can be easily addressed by using something like the "english lowercased name" as code. In fact that will help other systems because those subjects will be human-readable no matter if the target system doesn't know anything about GEM.
Finally we could opt-in to support multiple classification systems at the same time, in fact not all them are about "matters" but can be about other criteria, say difficulty, or audience or whatever (anything not having a related element in the DC specs is susceptible to be classified by multiple different criteria).
But that's another thing. Exclusively talking about "matter" classifications, I think my +1 goes to GEM and my +0.5 to UNESCO.
Ciao
