I know it may sound evasive, but it depends on 
what you are looking for in my opinion.There are 
so many wonderful collections of such a variety 
of anime. It's like asking which is the "best 
vitamin for a happy and healthy life?" Is it 
Miyazaki? Lodoss? Ah My Goddess? FY? DBZ? Gunnm? 
Esca.?, etc.,etc., you get my point?...And to 
leave out other anime (for those series that I 
didn't mention) would be a crime (*sorry*)So, 
maybe asking what is the "Best all around" 
collection (?), might be closer to reaching some 
consensus or statistical result for you I think. 
But it is still a tough call!! ;P

Let's start right here. Yann Stettler has 
a 'Great' collection cels. A wonderful assortment 
of precious and glorious cels, but then again I 
am a man. And he has a lot of women character 
cels so, I might be naturally biased :)
 Anyway, good luck with your documentary and 
success with finding "the best" ^-^
E.