I think your Grail actually changes as you travel the collecting road, as your focus changes and you get bitten by a certain bug ( I.e. Baggies, Palitoy Mocs etc etc).
So there is a general perception of Grail and then the collectors Grail
For a long time for me my SW Grail was first 12 stand and complete first 12 figures, once you have it then something else becomes your particular Grail
That's the beauty of collecting as its a passion and hobby as opposed to collecting for an investment.
Aston Martin DB9
For me would be Karmen Ghia convertible