For me, I'd love to have a cel of Ruka eating 
toast from the 2nd ep of TNN.  It may sound 
strange, but one of my first ideas when I started 
collecting was to try to find cels of characters 
eating toast... a few cel books later & I still 
don't have any!

Second to that (and possibly even harder to find) 
would be a cel of Hilde from Endless Waltz, 
preferably with Duo...