Here's why:
* Deep connection: They share a long history, having met at Camp Half-Blood as kids. Their bond is established early on, even before they realize their shared demigod heritage.
* Mutual trust and support: Throughout the book, they rely on each other for guidance, courage, and understanding. They share their fears and vulnerabilities, and always have each other's backs in dangerous situations.
* Shared goals: They work together to overcome challenges and complete the quest to clear Percy's name. They both have a strong sense of loyalty and responsibility to each other and their friends.
* Romantic undertones: Although not explicitly stated, there is a strong romantic tension between them that adds another layer to their friendship.
While Percy also has strong friendships with Grover and Luke (though Luke's motivations are ultimately revealed to be different), the depth and complexity of his relationship with Annabeth makes them the most significant friends in *The Lightning Thief*.