Spinda crest: Reverses stat changes (effect of Contrary) for every pokemon on the field and dizzy punch doubles in base power.
Kricketune crest: Makes its sound-based moves have perfect accuracy, its bug type moves are considered sound-based and perish song activates the turn after its used
Goodra crest: Sets up sticky web on switch-in, and absorbs sticky webs on your side of the field.
Budew crest: All its moves have +3 priority, the effect of destiny bond is always active for it and its immune to non-damaging moves.
Articuno crest: All its attacking moves lower the opponents speed by 1 stage, and defog has a 50% chance of freezing its target and lowers their speed by 1 stage also
Moltres crest: Upon switch-in, sunny day is set up for 3 turns, and opponents take 1/16 of their max hp in damage each turn while moltres is on the field.
Zapdos crest: Increases its special attack by one stage each turn, but the user faints after reaching +4 special attack.
Comfey crest: Heals the ally on switch-in by 25% of their hp, and it gains stab from grass type moves.
Electrode crest: The user will always live on 1 hp after taking a fatal blow from any hp range and explode after the turn ends.
Tropius crest: While the holder is on the field sunny day is active, and it will gain the effects of a random berry each turn.
Carbink crest: Gains +2 in both defense and sp.defense upon switching in, and its 'room' moves will last for 8 turns (trick room, wonder room etc.).
Purugly crest: While this pokemon is active the opponent(s) is considered under the effects of taunt and torment.
Thievul crest: While this pokemon is active the opponent's held items have no effect, with Thievul obtaining their effects.
Octillery crest: For this pokemon, Octazooka increases in base power to 100, always results in a critical hit and gains +1 priority.
Cramorant crest: The holder triggers the gulp missile effect after using any water-type move, not just dive, and the damage of gulp missile doubles.
Cursola crest: The holder draws in ghost and dark type moves, gaining +1 defense and sp.defense, and perish body only affects the opponent.
Pincurchin crest: The user's electric surge lasts 8 turns, and while this pokemon is active spikes deal double damage on the opponent.
Toucannon crest: For this pokemon, Beak Blast will deal stab fire type damage, has +1 priority, deals the damaging hit upon use and has a 100% chance to burn the target.
Seaking crest: This pokemon's speed is doubled and horn-based-attacks (megahorn, drill run etc.) increase in base power by 1.5x.
Pyukumuku crest: This pokemon's status moves have +1 priority, and innards out always deals damage equal to the holders max hp.
Wailord crest: This pokemon's attacks are calculated using its hp stat and the holder weighs double its current weight.
Delibird crest: For this pokemon, Present recieves +1 priority, will always deal 150 bp ice type damage when used on the opponent, and always heal 50% of your allies' max hp when used on an ally.
this post is a lot and i tried to make the crests as interesting as possible