When I think of the best homefield advantages in the NFL I think of Green Bay, Philadelphia, Denver, New England, and Seattle. Weather causes issues for opposing teams in GB & NE, the fans are nutts in Philly, Denver has the altitude issues, and as mentioned above the Seahawks have a stadium designed to hold in noise.
The Super Dome can get loud too, but I can't consider it one of the best when for many years that place was empty. The Georgia Dome can be loud too, but the Falcons haven't been there long enough to establish a mystique around the place and will be building a new stadium in the next few years.