Well I believe the general concensus is that VPI line axanthics keep their color the best. Although there can be exceptional versions of the other lines that look just as good and keep their color, just as there can be poor examples of the VPI., I think in General VPI is more known for consistently for keeping its colors better into adulthood.
There is also the much more recent Black Axanthic also from VPI, which seems to stay more jet black rather than the greys and silvers of the normal axanthics, tho that project would require a far greater chunk of money if you wanted a visual.