I don't think there's one fic I could recommend because my tastes have changed so much over the years. What I liked in 2005 or 2010 is probably not something I'd like now. Whenever I go through old delicious or LJ or AO3 bookmarks, I'm always disappointed when I reread a fic I once loved to find that I no longer enjoy it.

That being said, I don't think the following fic is the best fic I've ever read, but it's one that I've loved for years: never too late to be who you might have been, X-Men, Charles/Erik; http://archiveofourown.org/works/220402