For the entire Avenger fan clan they’ll know who Thor is. Thor is the ultimate Asguardian who can’t go a bit far without his hammer. Mjolnir . In the later part of the series it is shown that God of Thunder has another weapon of his choice and also shown as his favourite weapon, the axe called the Stormbreaker. Though the Mjolnir does not lose its importance to the audience and to Thor but Stormbreaker was deemed stronger.
In Thor’s first solo movie it was shown that Odin felt his own heir was not worthy and capable of Mjolnir so he claims that whosoever holds this hammer, if he be worthy, shall possess the power of Thor” but he eventually proves his power and picks it up. Mjolnir is not a normal weapon it is made of a breaking star and can also be used it to summon lightning, fly, or manipulate energy.
Hela was the one behind breaking Thor’s hammer in the movie Thor : Ragnork , but the God of Thunder learned to use his own powers and became sufficient. In the Endgame Thor thought that a weapon could kill Thanos and travelled to Nidavellir and traded the Stormbreaker with Eitri which according to him had more powers than Mjolnir. Thor then used the weapon in the battle of Wakanda to show his powers had upgraded and that he is stronger than before. Thor also used it to impale Thanos in the chest and later decapitated the Avengers’ most dangerous foe, but Mjolnir always served a bigger purpose
There has been no doubt shown by MCU that Stormbreaker is deadlier than Mjolnir. In the Endgame it is seen that he Thor couldn’t alone do with Mjolnir he had to combine with Stormbreaker. In the movies final battle. Whereas anyone in the battle field could pick up Stormbreaker but only the worthy ones could pick the Mjolnir . After the fight Thor kept the Stormbreaker because it was stronger but that did not mean it was worthier.
Thor had given Captain America his Mjolnir during the battle and after Thanos was defeated Captain America return it back to the dimension where it belonged as he thought that its esteem would be maintained there. Stormbreaker might be stronger and do more good to the character but in the MCU Mjolnir carries way more legacy and respect in the eyes of viewers and the characters. It has been a symbol of worthiness throughout the movie Endgame.