Many people say the term is overused lately, and even though i have my personal grails, i have to agree.
For me those are games i would love to have for differend reasons. They aren't necessarily rare and i don´t care what everbody else is currently after.
One week ago i bought a Missile Command. Not rare at all. Everybody has it, but to me it was a game high on my list. I also bought a Tutankham the same day and i didn´t know how much i love that game until i played it at home. This one will never leave. It somewhat became my ´grail´afterwards