I'm an animator in Los Angeles and I have inked 
my fair share of cels for my films.  I have found 
that Rapidograph pens ( you can buy them at 
different width tips ) work the best and dry 
quickest, fastest, and there is less a chance of 
them smearing.  Plus, if you are very careful- 
mistake lines can be *carefully* scratched off 
soon after the initial ink.

However, like Lady Zatara said, please consider 
carefully re-inking your cels- as line fading is 
much better than a damaged cel. Lots of my cels 
have line fades- I have some vintage Western 
Animation art.  But I NEVER re-ink the lines 
mostly because as an animator myself, I feel like 
it's tampering with an original piece of art.  If 
the paint on the Mona Lisa began to fade a little-
 would you take some paint and "fix it up"?  

It's a personal decision ( not putting down 
anyone who re-inks their cels ).  I just hate 
drawing over something that the original 
artists/animators created themselves.

Good luck :)