For me, a holy grail machine represents a game that was among the first few machines i had ever played that had a big impact on me. I have most of mine now.. KISS was THE holy grail because it was the first machine i ever played, I had little interest in pins until i played KISS for the first time, after that i was hooked and started looking for pins to play anywhere i could find them. The next few pins i played after that one were Stern Dracula, Gtb Buck Rogers, Stern Cosmic Princess, Stern Hot Hand (i own ALL of these now) 

SO my "new grail" titles which i played and enjoyed yrs ago are Gtb Haunted House, Gtb Countdown, Bally Playboy, Star Trek, Williams Gorgar are all games i'd like to own some day. There are a quite a few others too but i could go on for ages listing them