look at it like this.
Supermans only weakness used to be Kryptonite but as we saw in the ealry 90s, he can be beaten to death. it would just take someone immensely strong.
enter Goku.
so far, there are no limits on his strength. he can train and increase his power whereas superman, while incredibly strong, seems to have a limit.
I stand by my vote of Goku.