The strongest Dragon Ball Super fusion isn't between Goku and Vegeta.
During the Majin Buu series, Dragon Ball introduced the notion of fusion. Vegito and Gogeta
are two of the most powerful Dragon Ball characters ever presented. Vegito was
defeated in a duel against the Fused Zamasu in Dragon Ball Super. Goku had to call on Dragon
Ball's omnipotent monarch, Zeno, to remove this entity since it was so powerful and invincible.
The strongest fusion in Dragon Ball Super occurs from the collision of two pre-existing skills, not
from power levels. Fused Zamasu possesses both Gohan's and Kriss' skills, as well as being completely
immortal. This fusion is unavoidable, given that Goku Black is stronger than Kriss in breaking the Dragon Ball world.