That depends on how much the person you're gaming with is willing to stretch the canon. Strictly speaking, SSJ4 is the highest Super Saiyan transformation seen in the Dragonball canon universe, with the possible exception of the SSJ4 Fusion. That said, it should be noted that SSJ4 only appears in Dragonball GT, which was supervised by but not written by Akira Toriyama, making it a possibly non-canon transformation. If this be the case, the highest level of Super Saiyan seen is SSJ3, with the possible exception of Vegito, who was a fusion of two SSJ2 (Goku and Vegeta).