The hardest song I ever mastered on drums (I only play expert) would be either Hammer Smashed Face or Visions. I can get a good portion into Embedded but I eventually get tired, peter out and fail. I could probably pass Conquer All if I tried harder. This Is Exile and WAHNTHAC will need a lot of practice before I even think about passing them.
But Packet Flier, I don't believe I will ever be able to beat this.