Those Worthy to Wield Mjolnir

Alexander Power (5631)

Power is the leader of the Power Pack, a group of four, sibling superheroes. He lifted Mjolnir in defense of Thor during the battle with Jormungand, a serpent living under the World Tree.

Awesome Android aka Awesome Andy (616)

An android based off of Mr Fantastic, built by the Mad Thinker after he took over the Fantastic Four's tower. He was built with the ability to copy a personality trait and used this ability to copy Thor's nobility, allowing him to wield Mjolnir.

Beta Ray Bill (616)

Beta Ray Bill was proven worthy to lift Mjolnir and was able to defeat Thor in combat, making him the true wielder of Mjolnir. Odin had a copy of the hammer, named Stormbreaker, made and gave it to Beta in exchange for the original.

Natalia Romanov (23223)

Black Widow was granted worthiness while on a mission to recover Mjolnir in order to defeat Jormungand. Upon wielding it, she used the power to destroy Jormungand and everything else in sight, including herself.

Bor Burison (616)

Bor Burison, father of Odin and grandfather of Thor, was shown to be worthy after catching Mjolnir in battle, most likely a result of sharing the same blood.

Steve Rogers (616)

Captain America, specifically Steve Rogers, has been shown to be worthy on more than one occasion, making him one of the true wielders of Mjolnir.

Victor Mancha (14161)

Victor Mancha was determined worthy of wielding an alternate universe form of Mjolnir known as "Ultimate Mjolnir"

Conan the Barbarian (What-if? Thor battled Conan)

In "What if?: Thor battled Conan" Thor transferred ownership to Conan just before his death, allowing him to become worthy of wielding it.

Eric Masterson (616)

Eric Masterson became worthy after being merged with Thor. Later, after killing Loki, Thor would be banished by Odin, leaving only Masterson. This would be temporary, with Thor soon returning and Masterson being given Thunderstrike, a mace similar to Mjolnir

Jane Foster (616)

Following a battle with Nick Fury on the moon, Thor lost the ability to wield Mjolnir. It was abandoned there until Jane Foster came along and picked it up, making her the new Goddess of Thunder.

Loki Laufeyson (616)

In Axis, Thor and Loki, were caught under an inversion spell, resulting in Thor being a "villain" and Loki being a "hero." For the duration of the spell, Loki was able to wield Mjolnir and Thor was not.

Magni Thorson (3515)

As the son of Thor, Magni had a predetermined ability to wield Mjolnir and used it to defeat an evil version of Thor.

Odin

As the man who placed the worthiness enchantment on Mjolnir, he is the ultimate wielder of Mjolnir, capable of making or preventing anyone, including Thor, worthy of wielding it.

Roger Norvell (616)

Roger Norvell, a mortal film-maker brought to Asgard by Loki, attempted to get Lady Sif's attention by challenging Thor to battle. Odin, aware of Loki's plot to release Jormungand to kill Thor, gives Roger what he needs to defeat Thor, allowing him to claim the God of Thunder title so as to save his son from Jormungand.

Rogue (What if?: Rogue Possessed the Power of Thor)

Rogue became worthy of the hammer after using her ability to absorb and kill Thor.

Sarah Rogers (What if?: Brave New World)

Sarah Rogers, the daughter of Steve Rogers and Rogue, was shown to be both worthy and the owner of Mjolnir in Brave New World.

Ororo Munroe (616)

Munroe, better know by her nickname, Storm, was deemed worthy due to her gentle heart, fair nature, and determination for her fellow mutants. She would later use Mjolnir once more to shatter a cursed duplicate made by Loki, making her one of the few true wielders.

Kal-El (Crossover)

Superman was shown to be momentarily capable of wielding Mjolnir, loosing the ability a little bit later. Thor would explain to him that while Odin's enchantment was powerful, Odin wasn't a fool and most likely granted Superman temporary worthiness.

Additional mention: Wonder Woman was also able to wield Mjolnir properly as she shares the same traits as Superman plus being a warrior: http://media.comicbook.com/uploads1/2014/07/thor-wonder-woman-102933.jpg. However, she is quick to cast it aside, claiming to want a fair fight against Storm.

Vision (199999)

It is unknown if, in the MCU, the enchantment extends to artificial beings or not, however, Thor has acknowledged that Vision is definitely worthy of wielding the Hammer.

Honorable Mention: Wade Wilson

Deadpool was shown wielding a fake version of Mjolnir created by Loki. He knew it was fake and accepted it only because Loki would let him keep the hat.

Honorable Mention: Tony Stark

Stark was able to grab the hammer in space, however, the moment he entered the atmosphere, he was pinned to the "cabin in the sky" by the weight of it.

Honorable mention: Paramedic

A Paramedic, helping an injured Thor, was able to lift Mjolnir into his hand, something normally restricted by the enchantment.

Thor Odinson and all variants thereof

Thor Odinson is the rightful owner of Mjolnir after it is gifted to him by his father. Anyone with his spirit or anyone merged with him usually gains the ability to lift Mjolnir as a result.

Bonus: Mjolnir Engraving

The enchantment on Mjolnir is shown to change to "IF SHE BE WORTHY..." upon being wielded by Jane Foster, officially making her the true wielder and not a temporary one.

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