My vote is still for ADDAM'S FAMILY. It seems even casual pinball players are willing to pay big bucks to own it.
CACTUS CANYON always sells for ridiculous prices but it is too scarce to be truly overrated. Unfortunately for me, I have not played it so an honest critique is impossible. CHAMPION PUB also falls in this category. Maybe scarcity and popularity make for overrating?
MEDIEVAL MADNESS is DEFINITELY overrated too, but in my opinion, I think it has superior gameplay over ADDAM'S FAMILY which gives some merit to its lofty status.