Hey,
I'd have to say for all intensive purposes, The Shadow. I really enjoy the game, but keeping a couple of balls going while simultaneously playing the Battlefield in The Final Battle is rough. I would kinda sorta put The Simpsons Pinball Party in there, but I find it difficult more by the amount of stuff you have to do as opposed to what you actually have to do. None of the shots in and of themselves are particularly difficult, but the fact that there is SO much to do in a game to get to the end of it makes it hard. And Pin*Bot is a drain monster. Those would be the three I'd put in there.
Luke