Gen 3
Swampert crest, Moxie and summons rain for 3 turns on switch in.
Sceptile crest, Adaptability + Super Luck.
Blaziken crest, Technician if Blaziken doesnt have Speed boost, if it has this crest doesnt have any effect (or can act as a worst leftovers). (its here for the same reason infernape got a crest)
Absol crest, Fairy stab (just the boost, this doesnt make it dark/fairy) + Dark Aura.
Flygon crest, Bug type steelworker.
Gen 5
Klinklang crest, Boost atk and speed for weight reduction. (so it could spam autotomize)
Galvantula crest, Immune to entry hazards + setting up sticky web increases the user spatk by 1
Gen 6
Talonflame crest, Gen6 Gale Wings.
Aurorus crest, Invert it's weaknesses. So it would have a 4x resist to fighting / steel and 2x resist ground.
Tyrantrum crest, Defense = Special Defense. (its here for the same reason infernape got a crest)
Noivern crest, Aerilate, Speed = Atking stat.
Gourgeist crest, Flare Boost + atk and special atk are switched.
Trevenant crest, Regenerator + Hustle
Gen 8
Cursola crest, Soundproof (so it can ignore the perish body side effect) + special atk = defense (or spdef = def if its too op).
Sirfetchd crest, leek effect. (its here for the same reason infernape got a crest)
Falinks crest, Intrepid Sword + Moxie for speed.