I found an interesting website, called The Ultimate Sports List (www.tusl.com) that ranks the world's supposed 150 greatest sporting events. The Steelers-Browns rivalry makes the list, and is ranked #126 overall and #7 in the NFL (Super Bowl is #3 overall, #1 in NFL). Other NFL rivalries ranked are Packers-Bears at Lambeau, Cowboys-Redskins and Giants-Eagles.
Overall top sporting event in the world according to this site (which was supposedly done by fan voting)? The Summer Olympics.