Ah, this is a tough choice. No Guard is easily the most competitive ability on this list, but I don't think we'll learn as much from it as the other two. Weak Armor is the easiest to fit to the concept as the risk and reward are very clearly defined, as well as providing a kind of blueprint for stating CAP4. Illusion is a bit trickier to define the risk and reward, because it is less about how good the pokemon is, and more how good you are at using it. However, I think that isn't entirely what the concept is about, so i will go for Weak Armor
Weak Armor just seems too situational, plus the metagame will probably try too hard to adapt to it if it becomes a big selling point for this CAPmon. Illusion is very cool, though I feel it would be a bit too much of a diversion from the original concept.