AUS (Austin, TX). I fly a lot, and I have seen some pretty bad airports. The Austin airport is always quick through security, is always clean, and has reliable internet. And it is the only airport I have ever been to where the TSA agents are legitimately friendly and trying to get you through.
I fly in and out of Atlanta a lot, and the TSA agents there intentionally try to make your life difficult. I think the worst airport experience I had was in Newark, NJ, though. That airport was absolute trash.