First of all, cels are not my favorite thing to have 
autographed. For me, the best thing to sign would be a 
Shikiji(lit-Ceremonial Paper) Shikiji are almost square,
have board with a bit of gold leaf on one side, paper on
the other with a gold trim.  

It goes without saying that someone involved in creating the
anime would be a great candidate to sign your cel. 
Here's the order I'd rank people:
1-Character Designer(original)
Yoshitoshi Abe did the original character designs for Lain.
2-Character Designer(animation)
However, a character designer at the studio adapted Abe's
designs for animation purposes. 
However, some animation character designers do more than 
just adapt the originals to animation. Nobuteru Yuuki did
an excellent job of refining and adding more personality
to Izubuchi's designs for Lodoss Wars.  Granted they are not
Yuuki's original artwork, but he had a significant artistic
contribution to them. In those cases, animation character
designers can rank higher than the original designer.
3-Episode Director
The "Enshutsu" does the most work involved in coordinating
production. Of all the anime production jobs, this is 
probably the most demanding of them all. Checking animations
to see if they fit the director's standards, overseeing the
implimentation of director's(and his or her own) changes,
making sure all the cels look good enough for filming, 
keeping the filming on schedule, etc. 
Enshutsu isn't an overly respected job, but it's a very
important one.
4-Director
On an over all level and before the animation process 
starts, they oversee everything.  They don't necissarly 
spend as much time on the uber-detailed level that the
Enshutsu does.
5-Seiyuu. If I had a cel of Lina(and Lina only) and got
it signed(on a seperate layer of course) it would have to
be by Megumi Hayashibara, Lina's Seiyuu.  That's a given.
I would NOT pay a whole lot extra for a seiyuu sig.
If the cel looked nice anyway and the sig didn't interfere
with it, then I'd pay maybe $20 or so above what I see as
the value of the cel.  
If I thought that the seiyuu did a terrible job of voicing
the character, a seiyuu sign is out of the question.

Absolute dead last place-American Dub "Actors."
These people do not contribute to the creativity of an anime
at all. In some ways, they detract from it greatly with 
heinous voices, complete lack of acting, etc. While there 
are some good dubbers out there who capture the essence of
the character they do enough not to totally butcher it, 
many US VAs ahve turned some of my favorite anime into
deplorable experiences.  
If someone had a cel signed by a US VA, I'd totally pass it
up unless it was a really beautiful cel and the price was
right.  I would pay more for the cel if it was NOT signed by
a US VA.

That's my 2 pence/cents/yen/rupees