Thor is one of the most powerful superheroes in the Marvel Universe. There are a few characters who should potentially defeat him in a fight. 

The Phoenix Force is an exceedingly effective cosmic entity that has possessed several unique Marvel superheroes, including Jean Grey and Colossus. When possessed through the Phoenix Force, those characters have been able to defeat Thor. 

The Phoenix Force 

The Silver Surfer is a powerful alien who has been granted the power of the Power Cosmic by means of Galactus. 

Silver Surfer 

Thor's father, Odin, is even more powerful than Thor. He has considerable magical powers, and he's capable of wielding the Odinforce, a powerful cosmic weapon. 

Odin 

Galactus is a cosmic being who feeds on planets. He is incredibly powerful, and he has defeated Thor on numerous occasions. 

Galactus 

The Hulk is a fabulously strong superhero, and he has a healing factor that makes him difficult to kill. 

Hulk 

Thanos is a powerful Titan who wields the Infinity Gauntlet, a powerful cosmic weapon. 

Thanos 

Dr. Strange is the strongest sorcerer in the Marvel universe. who has access to a wide variety of magical abilities? 

Dr. Strange