Since the very beginning, Krillin has been Goku's best friend, and he is the strongest human in the Dragon Ball series.
He may not be as smart as Goku, but he is able to formulate incredibly well thought out plans on the spot.
Gohan and Goku used to train together in Dragon Ball Z. They spent one year, approximately one day in Earth time,
They have also displayed overwhelming strength both separately and in their fusion form, Gotenks.
Unlike his younger counterpart, Future Trunks has already matured and was trained by Gohan.
Uub is the reincarnation of Majin Buu and despite his size and age, he is one of the strongest humans in the show.
Due to him assisting Goku in his fight with Moro, Merus lost his status as an Angel trainee and became a mortal.
Goku did this in order to train with other strong fighters. Throughout their fights, Hit was able to take down Goku easily many times.
During the Granolah the Savior Saga, he was one of the main antagonists and gave Vegeta and Goku a run for their money.
He is another character who was once an immensely powerful enemy Goku had to struggle to take down