That's such a hard question; they're too many. Well one of the most famous one is Freak. But there's plenty of amazing troyler stories out there. Many make you laugh, cry, scream, get a personal attachment to the characters and intrigue you. I couldn't possibly pick a winner because every single one is a piece of art. There's also many types, e.g. Smut, angst, trigger, au etc so the 'best' fanfic would most likely be based on your preference. And, personally, I love (and sometimes hate) all the genres in fanfiction. I'm such a crap person to ask for a straight forward answer; sorry about that.