The castle toy! Probably the best toy in pinball. It's very mechanical and through player interaction reveals new shots. The only other toy that I can think of that is similar is the ship in POTC. The sounds, rules and music also play heavily into Medieval Madness's popularity.
I just wish the code for MMR could be made open source to allow home owners to create even deeper and unique rules for the game. The castle rules in MM can become repetitive after a while.