It is a pretty unique oil. While generally the brand of synthetic is a matter of personal preference, in this particular case nothing comes close as long as we are talking dog shifted non synchronized boxes.
If you find it hard to get, the BelRay and Mobil1 gear oils are good substitutes.
Best is always going to be an opinion. There is no way of proving what is the best. All of the various additives will do different things to help. Riding style, maintenance and other habits will effect how well the oil works for you just was much as the various additive packages.
That said, I use Mobil 1. I use it becuase I can buy it anywhere if I need it on the road for some reason, Wal-Mart, Kragen, Pep Boys, etc.