This is an interesting question...What do you mean by "holy grail?" Worth the most? Rarest? Which ones we all like the most?
In terms of rare and worth the most...The Benelli Volcano's, Speedway Silver Shadows, Broncco Mini Monster and perhaps a Bonanza would qualify...BUT, that doesnt mean they are the holy grail per se...
I own a 1970 Broncco TX-6 and for me, it's the holy grail! I like it more than my Volcano. I have one that is restored and one that has a big bore 80cc kit on it for an everyday bike. So it really depends on what you are looking for. $$$/worth isnt the only factor.