Grail set is obviously subjective.. though many people will have the same "Grail" sets.
I guess it is a favorite, nearly unobtainable set. May be nearly unobtainable due to quantity or funds, etc.
I feel like I have a couple, even though that may contradict what people consider Grail (as in one Holy Grail)
Obviously the RVCLs are one (and probably "the" one if it came down to it)... even though I had a sliver of hope that I might acquire Some I was fully accepting that I would never put a set of them together.
That said, it didn't stop me from digging and digging for a couple of years.
It was a combo of work and luck (some believe luck is created, at least partially, by effort).
Mapes are another set I considered a Grail set.. again got lucky and had the means to acquire a (pretty much... still need more $1s) playable set.
My 8v 43mm mixed set is kind of a Grail set.. but mostly due to the Horseshoe Gardena $500s (which I only know of 1 other rack in existence).
The other chips in the set are also great, but in more plentiful quantities.
Here is a little bean spilling.. 3 racks of my RVCL $5s came from Spinettis!



Sitting right under our noses for who knows how long!
Mikko told me he didn't remember when or where he got them (he only had the 3 racks of $5s and a few racks each of the $25 pri and sec)
It was a fluke that I found out he had them when discussing other chips with Mikko.