Who forged Stormbreaker for Thor?

Eitri forged Stormbreaker for Thor in the MCU.

Eitri is the king of the Dwarves of Nidavellir, a legendary weapons-maker portrayed by Peter Dinklage in Avengers: Infinity War. After Thanos attacks the dwarf forge and kills most of its inhabitants, he forces Eitri to create the Infinity Gauntlet. Thanos then kills the dwarves and permanently damages Eitri's hands so he cannot forge another weapon.

Thor travels to Nidavellir with Rocket and Groot seeking a weapon powerful enough to defeat Thanos. Eitri reignites the forge by opening the iris of a neutron star, then helps Thor create Stormbreaker. The weapon is made from uru, a magical metal associated with Asgardian weapons. Groot contributes the handle by sacrificing a piece of his arm.

The name Eitri can be confused with Brokk, Sindri, or Dvalin, all names associated with dwarven smiths in mythology and Marvel lore. In the MCU film, however, Eitri is the character who makes Stormbreaker. In the comics, Eitri is also a master forger, though Stormbreaker has a different history connected to Beta Ray Bill.