My centerpiece is my iminime Cowboy, and, for a long time, I'd say that Blondie was my grail piece, as I could never find one that really met my expectations.
Nowadays, though, as far as an "unattainable" grail goes, it's harder and harder to think of one. To be completely honest, this hobby embodies that spirit that "nothing is impossible." 5 years ago, I never would've thought there would be licensed 89 products, and a year ago, I never thought we'd be seeing anything from the Adam West series, as it had been in limbo for so long.
So, to think of that "unattainable figure," I'd probably have to think outside the box, and, even then, the only reason for it being unattainable would be purely financial. With that in mind, what the hell; my "grail" would probably be a line-up of TDK Trilogy figures from the likes of Adam and Kato commissioned to my specifications.
I just think that'd be the coolest thing in the world, to be able to pick any sort of variation for the main characters from Ra's to Two-Face and beyond.