Disclaimer: The Barton Creek Greenbelt is my favorite mountain bike trail. I might be a little biased, so bite me.
It may not be the best, or the most difficult trail in the world, but those are relative terms anyway. It's familiar, it feels like home, and it's mine. I know the different sections of the main trail and some (but definitely not all) of the backtrails. I know the sequence of the technical spots and which ones give me the most trouble, something my mind never fails to remind me of as I am trying to psych myself up to clear one. I know, from riding with others who know far more than me, when to call out before taking a blind corner or ascending/descending a blind hill.