Well, in my first month of collecting, I bought 
cels from Fushigi Yuug, X, and Sailor Moon, and 
was planning on grabbing whatever Utena I could 
find that I liked and afforded ^^;;  I currently 
have cels from 16 series, and although there are 
4 or 5 series I say I focus on more than the 
others, I generally just go by the rule of 
shopping for what I like.