Throughout its history, Goku has faced a really high amount of enemies. From when he was a child and collided with Pilaf until the new sagas of Dragon Ball Super where it has even had to do with the gods who control the universe, there have been many opponents who have become iconic.
The Dragon Ball Super anime recently brought two opponents into the canonical universe of Toriyama. The first is Black Goku, actually Zamasu, but that shocked the fans at the time since he showed himself with the same appearance as the protagonist. The second is Broly, actually in the franchise for some time but only thanks to the Dragon Ball Z feature films with original story.
Recently, Masako Nozawa released an interview for Dragon Ball: Dokkan Battle, a video game that is about to celebrate its fifth anniversary. The Japanese voice actress, Goku's historical voice, answered a question: "Who should face Goku again between Broly and Black Goku?". The voice actress thought about it and chose one of the two antagonists:"I'd say Broly. The bad guys are yin while the good guys are yang, either because of the fighting method or the way they think. So a clash between complete opposites would be the best, I think".
So for Masako Nozawa the choice is quite simple. At the moment instead Goku is facing Molo together with his companions in the manga of Dragon Ball Super, a clash that Masako Nozawa will surely have to voice over if Dragon Ball Super 2 arrives.