Oh man, first and foremost S Saturn (and any original uniform Saturn) comes to mind. I know of her appearing at the end of S, the beginning of Stars, in a transformation sequence in a videogame, and in a commercial for another videogame. But I have a few other very specific (and very expensive) kidney-grade dream cels:
http://sui.rubberslug.com/gallery/wish_ ... shID=23307 - Hotaru with the Saturn sign from episode 119 is my DREAM Hotaru. Not only is it my favorite episode, but that scene just leaves chills every single time I watch. It's such an iconic moment, and reveals her true identity to everyone.
http://sui.rubberslug.com/gallery/wish_ ... shID=23454 - I can only imagine how expensive an S OP Hotaru would be. I've never even SEEN one before.
http://sui.rubberslug.com/gallery/wish_ ... shID=23305 - Ever since finally watching Stars, I've been ITCHING for more Super Saturn cels. The two I have are absolutely spectacular and I wouldn't trade them for anything, but cels are like expensive potato chips after all.
http://sui.rubberslug.com/gallery/wish_ ... shID=23965 - Like hell I'd ever get such A1E cels, but that's what dreams are for. *u* And saving like mad.
http://sui.rubberslug.com/gallery/wish_ ... shID=23302 - Same for this. XD Cel from Saturn's only-ever transformation sequence, in the videogame SuperS - Various Emotion.
And last but certainly not least, ANY production artwork of the Pokemon Raikou, Entei, Suicune, Absol, and Land Forme (aka hedgehog) Shaymin. I have copy settei of all but Shaymin, but more and more I'm itching and hoping that one day I can finally actually own a piece of the Pokemon dearest to my heart. And of course Pokemon artwork is rare and what does get out is usually the same thing over and over.

But I never dreamed in a million years I'd own the Hotaru artwork I own even now, so who knows.
Here's my full (and quite lengthy and expensive) wishlist:
http://sui.rubberslug.com/gallery/wishlist.asp