Grass/Ghost
Cannot understand for the life of me why anyone would pick Poison/Dragon. Unless they really, really only want it to use one or two sets.
I wouldn't be happy with Electric/Poison, but at least it gives us something to work with. There are a number of different options available.
Grass/Ghost is easily the best remaining type. More or less a blank slate as far as weakness/resistance goes, a good few STAB options available to Sketch, multiple boosting moves are feasible, etc. I do wonder whether some people are simply looking at the typings and deciding which would be best on any random Pokemon, in terms of metagame effectiveness, rather than which will enable us to learn about the concept.
The concept won't fail if our CAPmon is too strong or too weak in the metagame. It will, however, fail if we don't learn anything from it, or create something that teaches us nothing we don't know already. See Cyclohm.