A few facts on this thread.
1) I am 42 - Spanks is just a bit over half my age by a few years. The guy is younger than average on here, so Pac Man, Defender, and Joust are just words to him (since those likely got converted to Time Pilot 84's or Street Fighters by the time he could reach the sticks).
2) TP 84 (aside from the unfortunate similarity to the acronym for toilet paper) is in my opinion a fine game although I prefer Time Pilot and Gyruss. I would LOVE to have a Time Pilot 84 - meaning I probably won't get one since Spanks is aggressive in collecting, and has no dependents. Me, I run the frikkin' Brady Bunch house over here.
Grail is a highly sought after item - in this case, a game you always wanted to have in your collection. For me, Tempest (ironically, got this from Spanks, who said something like "I never really played it and don't really like it") was a top grail along with Space Duel and Star Wars. None of these appeal to Spanks.
Ring King on the other hand does absolutely nothing for me, yet it was high on Spanks' list.
Everyone's grail will vary. If you lined up a ST:TNG, Twilight Zone, Addam's Family, and Medival Madness pin and said I could have one for free, I'd pick the TNG pin.
THAT is crazy, but that is grail.