It's hard to come up with specific guns that I would put on the level of "grail"... I have a small list of specific models that I have wanted all along, and I own them.
Everything else comes down to practical needs as far as firearms go.
There are a few calibers that still exist on the "I want one because" list.
.375 Cheytac
.408 Cheytac (Yes, I know the .375 is the same but better ballistically. Doesn't change the fact that I want one...)
Recently crossed the 6.5 Creedmoor off the list of wanted calibers.
ETA: I guess the only specific model I still want enough to have a "grail list" would be a Barrett M107A1.