I guess I just look for cels that are simply hard to find.
My wishlist looks something like this:

1. Macross (1984 movie or FB2012)
2. MegaZone 23 (part 1)
3. Top wo nerae!  GunBuster
4. Bubblegum Crisis (OVA)
5. Windaria (or possibly Leda -- I love Inomata art)
6. Vampire Princess Miyu (OVA)
7. Cowboy Bebop (Ein, Julia or Vicious)
8. Shoujo Kakumei Utena (TV or movie)
9. Fight! Iczer-One
10. Tenshi no Tamago (Angel's Egg)

Most of these shows are pretty old (at least ten years old),
so I can understand why they're pretty rare.  Still, I
know they're out there!