Yeah, kind of hard to have a structured conversation if we're just coming up with our own definitions of "grail." If you're going with the most "quality" figures, with great likenesses and reasonably accurate details, that are fairly iconic (not necessarily the most highly desired):
-DX Jack Sparrow (I don't own him as I don't care for the character or films, but probably the best Hot Toys figure I've seen)
-the Godfather (HT put out a real effort to do him as right as they could at the time. Sculpt, clothing, accessories, presentation, and source film are all top notch)
-Chris Reeve Superman (the movie super hero done very well)
-DX-12 Batman (best HT Batman figure I think, few QC problems, Nolan films are pretty culturally important at this point)
-Beat It Michael Jackson exclusive (best MJ sculpt released, everything is top notch. A bit under-appreciated because of the nature of HT collectors I think)
-MMS 136 Terminator (my one pet choice because of love of the film--the best Terminator figure released thus far IMO)
There are other very nice figures like various Iron Men (minus Stark sculpts), DX-11 Joker (though sculpts are a bit wonky), some of the Predators, and of course the less than iconic character that gets a homerun release like mime Joker or "Mandarin." Marty McFly has the potential to get on this list, but we'll see how the production figure comes out.