Very few heroes have saved the universe as many times as Dragon Ball's Son Goku, the Saiyan warrior who, in his quest for worthy opponents, has faced the strongest and baddest beings in existence. However, he's never stood in front of Thanos, the Mad Titan, Marvel's own universe-ending villain. Both are considered among the top fighters of their realities, but what would happen if they faced each other on the battlefield?
Both Goku and Thanos, while born mortals, have reached godlike levels of strength. Goku was able to perfect his body and spirit through training to the point of obtaining god ki, while Thanos used his genius-level intellect to augment his already impressive Eternal physiology through an array of technological and mystical instruments. While their paths have been completely different, Thanos and Goku do share a drive to constantly test themselves and their limits, and a never-give-up attitude fueled by immense willpower. For these reasons, the outcome of their hypothetical confrontation is hard to predict.
For this battle, assume that Goku has mastered the Super Sayian Blue transformation, but not the Ultra Instinct yet (he is still able to use it but only for a short amount of time). Thanos is at the peak of his power but does not use the Infinity Gauntlet, to make it a fair fight. The fight will come down to strength, speed and durability. Thanos is one of the strongest beings in the Marvel Universe. While fighting his son Thane, he destroys a whole planet by simply flying through it. On another occasion, he lifts and moves the Galactus Engine, the colossal propulsor unit that Ego the Living Planet uses to travel through space. Goku's strength is a bit harder to measure due to his transformations. In his normal form, Goku is shown as being able to lift no more than 40 tons. The Super Saiyan stages 1,2 and 3 multiply his strength by 50, 100 and 400 respectively, making it 16,000 tons. Super Saiyan Blue increases the power from the previous stage, the Super Saiyan God, 50 times, so Goku is able to lift a minimum of 800,000 tons, putting him easily on Thanos' same level.
In terms of speed, while Thanos is not known for his quickness, he is able to match in the Silver Surfer, who can travel thousands of light-years in a couple of seconds, close to the speed of light. Goku, however, is even more impressive. While fighting the God of Destruction Beerus, the shockwave from their blows is heard in another, adjacent universe, thus traveling several times faster than the speed of light. Both contestants are also capable of instant teleportation.
Durability is where Thanos gets a clear edge, however. He's survived the stomach fluids of Ego, which is capable of digesting stars. He has come back from a trip into a black hole and inside the Nexus of Realities. He is able to withstand the voice of Black Bolt from close range and can take several blasts of full Odinforce without even losing consciousness. Goku is also very resilient, having survived the fight with Beerus that almost destroys the whole of Universe 7. His weakness, however, is stamina. Each one of Goku's transformations and techniques, including the Ultra Instinct, consumes a great amount of energy, and the more damage he takes the less he is able to sustain them. Thanos, instead, seemingly never tires.
Physical feats, however, do not always determine the outcome of a battle, and that is the reason why Thanos would ultimately win. While Goku is a hardened warrior and a veteran of many battlefields, Thanos has thousands of years of experience, besides a genius-level intellect that allows him to deploy the best tactics in any situation. Goku is not exactly known for his wits and Thanos would easily outsmart him, which, together with his superior stamina, would give the Mad Titan the win in the end.