When you factor in the altitude, I think Denver is the hardest place to play. It is as loud as any stadium, but the weather can get crazy in a hurry and the altitude is a very difficult adjustment for any team. No dome should be considered the toughest place to play. I know from the Chargers perspective, they have beaten the Falcons and Colts in their dome stadiums when they were two very good teams, yet they have been owned by Arrowhead and Mile High for years. I don't think the Chargers have ever beaten both KC and Denver on the road in the same year in my lifetime.
aka KGB Chicken