If we are to include comic strips into proper literary fiction, they would open a whole new vista of amazing friends. But one friend who stands apart from the rest is
Peter Parker's best friend Harry Osbourne.
Peter and Harry have been friends even before Peter became Spider-man, and in a way their friendship has withstood the test of time.
I might even say, Peter failed many a times as a friend, stealing Harry's love-interest and even killing his father (though he never did any of it intentionally), but I have always found Harry true and precious. He always looked out for him and took everything in their friendships' stride.
He did revolt to avenge his father's death but it was only natural. Didn't Peter himself writhe in the same fire for the murderer of his uncle?
But what passes Harry as a true friend is that he didn't nurture ill-will for Peter, and when he needed him he did not fail him.
He is one friend every super-hero or otherwise would love to have.