Hmn..I don't have very many "rare" cels per say 
technical wise. But here are a few that may fit 
the description. :)

Hamlen movie OP cels are abound everywhere, but 
I've yet to see many chars other than Hamel. 
Here's a perfect shot of Raiel casting magic from 
the OP:



Another OP cel from a lesser known series, Gokodu-
kun Manyuki. Being lesser known makes the cels 
harder to find, so I was very thrilled to find 
this gorgeous opening cel of the main character, 
Gokudo:



Maybe one of my few FY cels that aren't found 
much anywhere else, a cel of Mitsukake using his 
seishi powers to heal Miaka's fever: