I would think "your grail" vs. "the grail" are two different beasts entirely. If we are going to talk about grail arcade games purely on a collectible level, Sunspot has to be it (original, not Sega asteroids clone). Short of prototype cabs, this is the rarest cab ever IIRC.
I think personal grails are titles that are likely unobtainable to that person for whatever reason...cost, rarity via location, etc. Then again, a good buddy of mine says when it comes to arcade collecting; unless you get a very lucky find, there are three categories; cheap, fast, and good...pick 2.