While "Grayson" is IMO the best fan film in existance, I feel the need to share the love for some others.
Keeping in the superhero theme, I recently watched "Worlds Finest" by Collora Studios and "Batman Dead End" by Montauk Films & Level 7. While not as good as Grayson, they are still pretty cool endeavors. I was quite surprised by where "Dead End" went.
Switching speeds to the genre that originally got me into fan films (Star Wars), there's "TROOPS." I would rate this as my 2nd fav. "Broken Allegiance" does a good job, but there's something about it that never sat right with me. "Revelations" is pretty good, potentially better than BA, but it was completely over-hyped by the time I saw it. I've downloaded "Seeds of Darkness" 1-3 and I've watched the first one. So far, so good. They haven't done much to attempt to botch it, but we'll see what happens in the other two parts.
So, if you've the bandwidth, I'd suggest checking out any of these fan films.