Storm Breaker

"With the iris closed, I can't heat the metal."
"How long will it take to heat it?"
"A few minutes, maybe more. Why?"
"I'm gonna hold it open."
"That's suicide."
"So is facing Thanos without that axe.
                                         
                                 : Eitri and Thor

Storm Breaker

The Stormbreaker is one of the most dangerous weapon in the MCU, was able to withstand the full might of all six Infinity Stones as we had seen in Avengers: Infinity War. But in the comics, Thor never lost his beloved Mjolnir. Then who really owned the Stormbreaker?

Beta-Ray-Bill

It was this guy, Beta Ray Bill. In the comics, the Stormbreaker was created by Odin as a weapon for Beta-Ray-Bill, after he returned Mjølnir to Thor. Thor and Bill battled after the former crashed into the latter's ship. Bill emerged victorious and was granted his opponent's hammer as a prize. Although the rightful victor, Bill began to have some second thoughts about taking it from the one who wielded it for so long.

To make up for this, Odin -- who oversaw the match -- decided to have a new weapon made specifically for Bill. It would essentially be the same as Mjolnir, it just wouldn't be Thor's Mjolnir. With that, Stormbreaker was born. With this, it's pretty clear that Thor is beatable.

Most of you must not have noticed that Beta Ray Bill was given an Easter egg cameo in Thor Ragnarok. His statue is seen below Hulks which proves that Bill was also the Grandmaster's favourite at the contest of champions.