I'd be interested in hearing other answers, but 
this is what I gather from this very informative 
website:

http://sockfairy.com/scott/articles/jobs/animejob.
htm

1.  The director of a particular episode does the 
storyboard, a very rough realization of 
what the show will look like.  These seem to be 
very rare, at least in autograph form, though 
I've seen duplicated storyboards that evidently 
are used by the workers in the stages below.

2.  The character designer will do a 
settei, or model pack, a series of 
drawings of the main characters in their costumes 
that others will draw on.  Images from the 
settei also get duplicated and passed 
around to the workers in the stages below.

3.  The key animators (or gengamen) will work 
from both of these to produce rough layout 
drawings, then a series of preliminary key 
gengas. (These are usually on white paper.) 

4.  The director, animation supervisor, or 
character designer will produce the 
shuusei or "correction" gengas.  (These 
are usually on light yellow or light green paper.)

5.  A team of "inbetweeners" will then produce 
the dougas, which are simple outlines 
drawn from the shuusei gengas, along with 
new sketches that provide continuity between the 
keys.

6.  These get photocopied onto the acetate sheets 
that get painted and turn into the cels.

This isn't complete, because I don't know where 
the bundle of duplicated sheets that I describe a 
few items above fit in.  Clearly it's based on 
the material produced in #3 ... but for whom and 
for what purpose?

Clearly there should be a more comprehensive and 
regular system of identifying these paper 
materials so we all know what we have and what 
its significance is.

As for for their comparative value ... I 
leave that to more experienced hands, though I 
would expect that original storyboards would be 
the most precious, and the shuusei gengas 
next.