I find STh gets repetitive and boring. Played about 30+ games on it, but the game was set up pretty easy to be honest. Afm gets boring quicker. Mm has the castle, which can keep me somewhat engaged.
Shadow, on the other hand, plays like no other machine, and the shots are a great mix. I love the upper pf extra ball carrot, the ball locks, the Khan super jackpot etc etc. His best game by a looooooong shot. Never gets old at all.