Captain America & Thor’s Mjölnir (hammer)!
According to a fan theory, which I believe in, Captain America is closest to being worthy among all The Avengers because he’s the most innocent, truthful, straightforward, courageous & kind. That’s why he could move Mjölnir.
But why couldn’t he lift it if he’s so good. Well, he is good (in fact better than all the rest!) but still not good enough.
The reason Capt. Steve Rogers is not fully worthy is because he wants war. He loves war. He wants to fight war. He is purpose built for war. A super soldier! He wanted to be in war so badly that he applied for it 6 times! What is Captain America without a war? In Avengers: Age of Ultron when Commander Maria Hill was telling him about Strucker & his experiments, he wanted to have a war with them. She said, “We are not at war Captain.” He said, “But they are!” He always sees war.
When Thor wanted war & waged war on Frost Giants, he lost all his powers & ability to lift Mjölnir as a punishment. His powers only came back when he decided to end the war by surrendering & sacrificing his life. Mjölnir came back to him when he decided not to fight anymore for the greater good of others.
Captain America never surrenders. He considers himself to be the best & bravest of all warriors. He thinks he can always win because in his mind he is always on the right side. He would rather keep on fighting than surrendering for peace. He is so much engulfed into his soldier mentality that he doesn’t realise that to surrender is a virtue. And because of the lack of this virtue, he is not fully worthy, yet. But he is very close.
Thor learned this virtue the hard way.