Chesnaught crest: Gains the ability filter, to help reduce damage from super effective moves; given how it has 6 weaknesses. This would allow it to tank those type of moves better.
Delphox crest: Increases the odds of secondary effects of actually happening (like burn or special defence drop for example) by 10%~20% roughly. Delphox can learn moves like flamethrower, Psychic and Shadow ball for example. Getting increased odds of dropping sp.def and burning opponents sounds like a good plan.
Greninja crest: Boost stab moves from the usual 1.5 multiplier to a 1.75 or 2.0 multiplier. This will allow Geninja to hit harder and with the ability protean, this crest will be extremely useful.