Yamoshi was the first ... go Super Saiyan. But that is not the case. Sure, he learns it and is able to wield it for a certain period of time, but somebody else gained this ability long before Goku ...
I am truth. Ally to good! Nightmare to you! When Goku went Super Saiyan for the very first time, there was nothing in the entire universe that was on the same level of strength as him at that point.
Even at the end of the movie, where Goku shattered his limits and achieved his brand-new Super Saiyan God form ... under a damaged Mosco the entire time. By drawing everyone to Champa's location ...