One great Mikasa x Sasha fanfiction could be 'The Bond Beyond the Walls'. It delves into their relationship in a post - apocalyptic world, exploring how their different personalities complement each other. They face challenges together and their friendship gradually evolves into something deeper.
You can try Archive of Our Own (AO3). It has a large collection of fanfictions across various fandoms, including Attack on Titan, so there's a high chance of finding some great Mikasa x Sasha fanfictions there.
Annie is often depicted as a more reserved and calculating character. She keeps her emotions in check and is very strategic in her actions. Mikasa, on the contrary, is more passionate and driven by her love and loyalty. She is protective and doesn't hesitate to jump into action when needed.