"That was my best friend. And you just killed him!" This might be the very first time Superman explicitly states that they're best friends.
Batman and Superman's friendship. It's a lot more explicit in the book rather than the movie, because much of the text box narration is taken up by their inner monologues - most of which are both of them thinking about how much the other inspires them and how honored they are to work with each other. Lots of things they would likely never say out loud, but all highly heartwarming.