Normally, people will say Registered Magnums
For me the Holy Grail is a three inch model 19.
Can't argue with the Registered Magnum or the 3" M19. I'd throw in a ANIB K-32 Target Masterpiece in there as well.
Gotta point out that for as rare as these models are, they are obtainable. In contrast, Supica & Nahas refer to a number of revolvers S&W reportedly made, but are so rare as to make valuation guesstimates difficult. If "Holy Grail" discounts utility and refers only to availability of guns actually made, these phantom models must surely place higher on the Grail Scale.