I would not call these ‘Grail’ pins, as they are not rare or special
But my collection will not be complete with out two Williams EM machines; Grand Prix and Space Odyssey
The two games that I first remember playing

Hard to top Fathom as a rare pin, Pete 

Rocket Ship would be near impossible to find (I know of one local to me).

A true "Holy Grail" would be like finding the fountain of youth - or the pinball equivalent - which could be a Fathom, or a Mermaid.

Grand Prix and Space Odyssey are EASY to locate and would not be much of a challenge. They are more "sentimental" machines for you  ?