Everyone has that one cel/sketch/etc. in their gallery that they can't believe they own. It may not be your ultimate wishlist item. You could still be waiting for that one. But I'm sure everyone has a piece of animation art they can't believe they have, even if you are just starting collecting. When I first started collecting, I just couldn't believe I owned a Sailor Moon cel of Sailor Moon.

So show me your HOLY GRAIL!
Mine in 2006 was "The Syaoran." But I've been lucky since them, so now I'd have to pick:
CAUTION!!!! MEGA-ROZEN-MAIDEN-PLOT-SPOILER!!!!! (But dang, you know you gotta go look at it...)
That was a moment that I still remember from the first time I watched this series through: I was surprised, both by the action and by my emotional reaction. It wasn't what I expected, but I felt it was exactly right as the climax to the action in this series.
When I began collecting, I got some nice art from Rozen Maiden, and thought, golly, wouldn't it be nice to have the genga/douga from "that" split-second. Then I laughed and laughed and laughed at the absurdity of the idea. After all, that was the basis for one of the rilezu painted from the series, so whoever paid top dollar for that piece of paint-and-plastic has the original animating sketches in his or her top drawer. Right?
And yet ... one day ...