Rarest ones:
Splatter and camo clacks, transparent clacks, wingnuts (good luck finding one of those), Red darth vader (if you can convince Ripster to sell it to you), 66hz clack, metallic clacks (as far as I know silver one with red eyes is the rarest), and all tri color skulls (GH, canada, skeletor...), 420, darth vader... Expect paying alot for there ones.
The most common ones:
hack orange, 3d, hardcore white, mint green...
Something between:
all other single color skulls, F2's, octoberfest, julysicle, candy corn...
This is all I know