Here are crest that i suggest
Furret Crest-gives Furret protean and boosts it's speed by 0.5 when entering battle AND switches it's attack with sp. attack
Mightyena crest-gives dark moves priority and 0.5 boost for attack and speed after KOing pokemon
Greninja crest-water shuriken power buff from 15 to 25 and water shuriken always hits 5 times (like skill link but only on one move),also it would be cool if
CHARIZARD GOT CREST (i know that won't happen cuz megas are coming in v13 )
I realized that only 2 people suggested Mightyena before me