Interesting... a bit different from the usual 
question.  "Best" cel, instead of favorite?

Well, let's see... I generally don't collect what 
the hobbyists refer to as "A-level cels", which is 
to say that I have very few backgrounded images, 
and many of my character cels aren't of the 
classic portrait type.  Of the few backgrounded 
cels that I do have, I've tended to favor 
expansive settings with small character images -- 
nice so that the background can be appreciated, 
but not particularly helpful when choosing a 
"best" cel.

There's also the damage issue.  Some of my 
favorite cels, which I would also count among my 
most beautiful, have issues with line fading or 
paint flaking.  It's just a common hazard of cel 
collecting -- we do our best to preserve them, but 
there's nothing to be done about the treatment 
they received before we acquired them.

I suppose, with all of this in mind, I would have 
to make an unusual choice.  The following cel is 
in nearly pristine condition, of a primary 
character, from a very pivotal scene, and conveys 
the emotion of the scene quite well.  In addition, 
the frame which it was used to film has been 
featured on a trading card, as well as in 
promotional materials for the movie and the 
trailers which appear on related DVDs.

So, technically speaking, I suppose this would be 
my best cel.  ^_^

Sorry for rambling... but thanks for posing a very 
thought-provoking question!