Most Popular That I've seen from looking at a 
large number of online galleries:
Tenchi Muyo 
Dragon Ball Z (Ack! ooops! Sorry to all DBZ 
Collectors)
Fushigi Yuugi 

Up & Coming:
Card Captor Sakura (many people have a few cels 
now)
Gundam Wing

Confession:
I collected Magic Knights Rayearth because the 
cels were reasonably priced & I loved the Clamp 
character designs. Its a series thats popular 
with a few collectors so prices have not 
skyrocketted. Those collectors who view 
visitation stats on their websites can tell you 
what is really popular.

My Tenchi page has been viewed 4x more than my 
Rayearth page. (Next in popularity is Utena and 
Fushigi Yuugi).

With a tight budget (not a small one. A 
reasonably sized disciplined one). I tend to 
focus on individual characters that I "see-want-
must-have-NOW". For instance I shelled out 
reedeekulous amounts for cels in Utena but 
remained very conservative in most of my other 
series (A few Ryoko & Bebop cels excepted). Key 
scenes and nice facial shots are very important. 

Having said that if I have a few cels of a single 
character eg Ryoko/Lina Inverse I tend to look 
for different facial expressions/outfits/scenes 
etc. That is unfortunately a luxury I cannot 
afford for really expensive characters eg Juri 
from Utena.

Rgds

Alex
(In vain hope someone - ANYONE has a Sonoda Keiko 
or Tatsuki Shiori "Black Rose" cel from Utena)