There are three of those for me.  

The first, please don't groan, is Fushigi Yuugi.  
I don't think I will ever find that one Hotohori 
cel that 'speaks' to me that I can afford.

The next one is actually a PS1 video game, 
Xenogears. There are a few anime sequences in it 
to advance the story, and I would love a cel of 
Citan (or Shitan, for your purists!) from one of 
the only three times he actually appears in them.

The final one might be a little more obscure.  I 
am certain it is the most costly of the bunch.  I 
would love a cel of the winged girl from the 
Miyazaki video "On Your Mark" by Chage and Aske.  
I think that is the name of the group who sings 
the song in the video.

So there it is, though there are always other 
shows and cels I admire, my top, top, top ones!

^_^