I think this should belong in the Musicians subforum, BUT;
You ask in the title for the hardest song to play yet in the poll you ask which one is hardest for drums. I don't know that, but I know it for guitar:
For me, it would be Blind Faith, because the unison is fucking difficult. The Glass Prison is also a good contender, but that's mainly because of the stamina needed to play it all the way through and the solo around 10:25 contains a stupidly hard and fast run.