As of 2020 (Generation 8):
Bug: Fury Cutter at max power (160), otherwise Megahorn (120). Fury Cutter should not be used - Heracross, Rhyperior, Samurott, and Scolipede make great use of Megahorn.
Dark: Foul Play (95) - Umbreon
Dragon: Eternabeam (160) - Eternatus. Strongest non-signature is Draco Meteor (130) and is popular on numerous Dragon-types.
Electric: Zekrom's Bolt Strike (130).* The strongest non-signature Electric type attack is Zap Cannon (120), but has dangerously low 50% accuracy. Electro Ball can be as powerful as base 150 if you're fast enough, but usually Thunderbolt, Discharge, or even Thunder are better options.
Fairy: Magearna's Fleur Cannon (130). The strongest non-signature Fairy move is Misty Explosion (100), but it is rarely seen.
Fighting: Reversal at very low HP (200), otherwise Focus Punch (150) - Dugtrio and Substitute Breloom builds, respectively
Fire: V-create (180) - Victini.* The strongest non-signature Fire moves are Blast Burn and Eruption at 100% (150), the former of which can only be learned by starter Pokemon and are not recommended for use.
Flying: Sky Attack (140) - Power Herb Hawlucha builds.
Ghost: A tie between Giratina's Shadow Force* and Ghost Rider Calyrex's Astral Barrage (120). The strongest non-signature Ghost type attack is Poltergeist (110), which is especially useful in competitive environments but weak in-game.
Grass: Frenzy Plant (150) - Grass starters, but it shouldn't be used. Leaf Storm (130) is the highest non-signature, and most popular on Serperior.
Ground: Magnitude 10 (150), otherwise it is a Bone Rush hitting 5 times (125), followed by Groudon's signature move Precipice Blades (120), and next is Earthquake (100) - Magnitude and Bone Rush shouldn't be used, but lots of Pokemon abuse EQ.
Ice: Ice Ball on its fifth hit, otherwise Freeze Shock/Ice Burn (140) - Ice Ball shouldn't be used, but Kyurem for the latter two. Besides the signature moves, Blizzard takes the spot of the strongest Ice type move (110, formerly 120).
Normal: Explosion (250) - Not really the best move ever, but is sometimes used on suicide leads.
Poison: A tie between Belch and Gunk Shot (120) - Use the latter with caution, but Pokemon like Muk and Arbok can abuse it for extra power. Greninja learns this move and is a potent answer to Fairies with Protean. Do not use Belch.
Psychic: Prismatic Laser (160) - Necrozma, though recharge makes it not worth investing in. The strongest non-signature Psychic attack is Psychic (90).
Rock: Head Smash/Rock Wrecker (150) - Rampardos, Aggron, Tyrantrum, Relicanth, and perhaps even Steelix for Head Smash; Rock Wrecker generally should not be used.
Steel: A tie between Jirachi's signature move Doom Desire (140) and Steel Beam, which can be situationally useful but used with extreme caution due to heavy recoil.
Water: Hydro Cannon/Water Spout at 100% HP (150) - Hydro Cannon shouldn't be used and Kyogre for Water Spout
Exlcuding OHKO Atacks, listed in no real order. Also note that I did not post like, every Pokemon that can learn them, just some notable users.
- Although Bolt Strike, V-Create, and Shadow Force are signature moves, they were given to Victini, Rayquaza, and Arceus, respectively, in special distribution events.
