Ghost Recon Advanced Warfighter 2. I've heard people saying the first one is the hardest, but I had more problems with this. I think I just spent some 2~3 hours trying that final mission again and again. Talk about difficulty spike.
Games finished so far:
Vampire the Masquerade: Bloodlines, Darwinia, Dark Messiah of Might & Magic, Shadowgrounds, Braid, And Yet it Moves, Modern Warfare 2, Shank, Dead Space, Kane & Lynch 2, Modern Warfare 3, Rogue Trooper, Depths of Peril, Portal 1 and 2, Ghost Recon Advanced Warfighter 1 and 2 (PC versions).