I gotta be honest, the 800 does sting more than some of the other events, but that doesn't necessarily make it difficult, you just have to be prepared for the effort.
I think most difficult has to go to the 400 hurdles. The 100 hurdles are difficult because of the learning curve with technique. But a solid 400 already locks your legs up quite a bit, and adding the required athleticism of covering 10 hurdles around a track with near-perfect technique while running as fast as you can for 400 meters knocks a lot of really talented people out.