I am aware there's already a selected answer, but I'd like to add my own two cents. I agree with everything the selected answer says, but there's some more Pokemon I think are also very solid.
Noctowl
Hoothoot can be found on the very first routes, and Noctowl can be found in several places in the Wild Area, so it's pretty common. Noctowl is surprisingly good, with access to moves like Moonblast, Hypnosis, Shadow Ball, and Psychic.
Noctowl @ Silk Scarf / Sharp Beak
Ability: Tinted Lens if you can get it, Insomnia if not
- Hyper Voice
- Moonblast
- Shadow Ball
- Air Slash / Hurricane / Hypnosis
Diggersby
Buneary and Diggersby are both incredibly common in the Wild Area, it should be easy to pick one up. Huge Power is an amazing Ability, and Diggersby is still good even if you can't get it.
Diggersby @ Silk Scarf / Soft Sand / Choice Band
Ability: Huge Power if you can get it, Pickup if not
- Earthquake
- Body Slam
- Fire Punch / Ice Punch / Thunder Punch
- Brick Break / U-turn / any other physical move
Bewear
Stufful can be picked up in the Wild Area at the entrance of Motostoke, it's quite common. Bewear has an excellent Attack stat that'll put a decent dent in any opponent.
Bewear @ Silk Scarf / Black Belt / Assault Vest
Ability: Fluffy
- Brick Break / Close Combat / Drain Punch
- Thrash / Strength / Body Slam
- Iron Head
- Darkest Lariat
Snorlax
Snorlax can be obtained by the bridge in the Wild Area to the right of the Motostoke entrance, but you'll need a couple Badges before you can get it. It's slow, but it hits like a truck, making it worth your while. It has a gigantic movepool, so you can use basically anything on it.
Snorlax @ Expert Belt / Leftovers / Life Orb
Ability: Thick Fat
- Body Slam
- Hammer Arm
- High Horsepower / Earthquake
- Darkest Lariat
These Pokemon are obtainable if you have the DLC. You'd have to go a bit out of your way to get them, or trade, but they're quite good.
Lopunny
Lopunny can be a pain to get, as it's a friendship evolution, but Buneary is incredibly common in the Isle of Armor. It hits decently hard and it's got an impressive Speed stat.
Lopunny @ anything
Ability: Cute Charm
- Close Combat / High Jump Kick
- Headbutt / Quick Attack
- Play Rough
- U-turn / an elemental punch
Bouffalant
Bouffalant, while slow, has excellent totals in HP, Attack, Defense, and Special Defense. It can be found in the Soothing Wetlands, so you'll need to go through a small bit of the Isle of Armor story.
Bouffalant @ Life Orb / Silk Scarf / anything
Ability: Sap Sipper / Reckless
- Head Charge
- Throat Chop
- Earthquake / Stomping Tantrum / High Horsepower
- Close Combat / Zen Headbutt / any other physical move
Tauros
Tauros is basically a faster, slightly frailer Bouffalant. It packs quite the punch with a base Attack stat of 100. It can be found in the Isle of Armor.
Tauros @ anything
Ability: Intimidate
- Body Slam / Thrash
- Earthquake
- Throat Chop
- Zen Headbutt
Porygon-Z
Porygon-Z is a very good special attacker, but it does require trading. If that's not an issue, you should probably use it. Hyde gives you a Porygon in the Maser Dojo.
Porygon-Z @ Wise Glasses
Ability: Download / Adaptability
- Psychic / Psyshock
- Shadow Ball / Tri Attack
- Ice Beam
- Thunderbolt
Linoone-H
Linoone-H is probably a bit weaker than a lot of other entries on this list, but it can still be useful. Belly Drum is fearsome, even if not always needed in-game. You can get one from Max Adventures in the Crown Tundra.
Linoone @ Sitrus Berry / anything else
Ability: Pickup / Gluttony
- Belly Drum
- Extreme Speed
- Throat Chop
- Stomping Tantrum
Hope I helped!
