Everyone defines it different, yes. I suppose if a grail meant the one game you had to have over all others mine would be Gorf. It was easy to find. However, I had a really hard time finding a Sinistar and FINALLY got one; after dreaming about having one since 1983. So its a different kind of grail...
Red Tent for sure. I don't own one because they are damn near impossible to find in CA. Be it, I've only been in the hobby for about 6 months, but still. I don't think I've ever seen one pop up. I bet there's one guy in CA that has them all stashed in a warehouse never to be seen again. Like the closing scene from Raiders of the Lost Ark.
Other than that, a Star Wars UR and a dedicated SFII:WW.
my grail is a little different. a collection of everything williams including both video game and pinball, 70's to 90s (and time pilot and pacman), including a shed big enough to hold them all. got the acreage, just dont have the roomage, housage or sheddage (yet).