Well into my second hour of Thief 2's final mission, Sabotage at Soulforge. I swear, that one mission is at least twice, if not three times as long as Life of the Party or Masks. The only reason I can do Life of the Party in under an hour is because I played that mission so often that I have almost the entire map, the location of nearly every secret, key, enemy, and bystander memorized.
Nothing is as satisfying as your final, epic battle being you single-handedly taking on an extremely well-guarded factory/cathedral that's staffed by an army of security personnel, and dozens of security machines. Storming the place is fun, but being able to finish the big boss while being completely undetected is almost orgasmic.