49. Thor vs. Beta Ray Bill
Beta Ray Bill was a member of a race of people called the Korbinites, a people who were driven to near extinction when their galaxy was ravaged by the evil Asgardian creature Surtur. When Bill’s spaceship enters Earth’s orbit, Thor is sent by SHIELD to investigate. Bill’s ship detects Thor as being related to Surtur (they’re both Norse) and attacks.
Bill fights Thor pretty well, and in the melee, Thor loses his hammer and reverts back to Don Blake. Bill picks up Blake’s cane and accidentally taps it, becoming a version of Thor!
Odin then determines Bill and Thor should fight for who gets to be the wielder of Mjolnir. They have a brutal battle that ends with both being knocked unconscious - Bill wins by being the first to regain consciousness (due to his alien DNA he recovers easier in the circumstances they found themselves in - they were in Bill’s neck of the woods).
While he does not make Bill Thor, per se, Odin does create a brand NEW magic hammer just for Bill called Stormbreaker.
Bill has basically been an alien version of Thor from that point on. Walt Simonson (writer/artist on Thor at the time) felt Beta Ray Bill would be just the fresh start the book needed, so he featured Bill in his first storyline on Thor, #337-339. Fans quickly responded favorably to the off-beat new character.