I am curious. We all know Goku is a powerful opponent, but he has flaws in his character and he is not unbeatable. With that said, there are opponents who beat him, and those that he loses too. He would beat the HST, but would lose to Sailor Moon and beyond..... BUT! That is not what I want to know in this thread.
What I want to know is.... who would be Goku's greatest challenge? A fighter who is his equal in every single way. That neither he, nor the opponent, could get an upper hand against one another.
People say that Superman is his greatest equal, and I disagree. I think Superman is well above Goku, but I, nor anyone, is going to bring Superman up. It is forbidden. We all know how Superman and Goku in the same thread gets way too out of hand.
Goku's greatest challenge must be from the anime universe. Any anime you can think of. I would like to know who would be the Yang to Goku's Yin in anime.