Held Items
Razor Claw (+1)
Scope Lens (+1)
Leek (for Farfetch'd or Sirfetch'd only; +2)
Lucky Punch (for Chansey only; +2)
Abilities
Super Luck (+1)
Battle Armor (Pokemon cannot be hit by critical hits)
Shell Armor (Pokemon cannot be hit by critical hits)
Triggered
G-Max Chi Strike (+1)
Focus Energy (+2)
Dire Hit (+2)
Dire Hit 2 (+2)
Lansat Berry (+2)
Z-Foresight (+2)
Z-Sleep Talk (+2)
Z-Tailwind (+2)
Z-Acupressure (+2)
Z-Heart Swap (+2)
Dire Hit 3 (+3)
These moves have an increased critical hit ratio. If the move is Storm Throw, Frost Breath, Zippy Zap, Surging Strikes, Wicked Blow, or Flower Trick, it will always be a critical hit. Using Laser Focus guarantees the next move to be a critical hit. Lucky Chant protects the user from critical hits. The Ability Merciless guarantees critical hits on all poisoned Pokemon. If a Pokemon's Affection is high, it has a higher chance of scoring a critical hit.
Critical hit (Bulbapedia)
I believe that's everything. Hope I helped!