In Pokemon singleplayer campaigns, attack stats (and STAB moves in older games where some Pokemon didn't have powerful STAB moves) are usually the most important thing that determine how useful a Pokemon is, and types, coverage moves, and status moves often matter as well. There are also things that don't matter in competitive, like when you can obtain and evolve each Pokemon. Here are a few Pokemon that have low stats and are still useful in singleplayer.
XY
Vivillon: Its stats are objectively bad but its special attack is just high enough to work with. Sleep powder and quiver dance are 2 of the best status moves in the game, especially with compound eyes. Compound eyes hurricane is also powerful.
Zangoose: It has low stats but its stat distribution is almost perfect. It learns swords dance early, return is a very strong and spammable STAB move, and it gets good coverage moves like shadow claw, dig, and close combat.
Sawk: Its stat spread is similar to Zangoose. It learns brick break, bulk up, and close combat by level up and can learn useful coverage moves like payback, rock slide, and poison jab.
Throh: It does almost everything that Sawk does, but it's a bit worse because of its lower physical attack and speed.
Azumarill also technically has low stats but it's useful because huge power.
ORAS
Beautifly: It evolves early and learns air cutter and silver wind not too long afterward. It's worse in the mid-game, but it gets better later thanks to bug buzz, quiver dance, and a few other moves like giga drain and shadow ball.
Breloom: It has high physical attack, enough speed, good offensive typing, and bulk up. It learns a few other useful status moves, like stun spore and leech seed.
Hariyama: Unlike most of the other setup users on the list, Hariyama uses its high HP and bulk up's physical defense boosts to tank attacks. Learning force palm and knock off at very low levels also helps.
Azumarill and Mawile also technically have low status but they're useful because huge power.
USUM
Gumshoos: The totem Gumshoos skips its evolution and the early game. It has high physical attack, strong jaw, and powerful moves like hyper fang, crunch, and earthquake.
Pelipper: Early Flyinium Z makes a lot of flying Pokemon broken. Pelipper also evolves early and has drizzle powering up its water attacks.
Hariyama: This Pokemon doesn't change much from gen 6. It uses its HP to tank hits and bulk up to sweep.
Oricorio: This is likely the second best Flyinium Z abuser in the game, after Hawlucha. It outspeeds most things, has good defensive typing and air cutter in the early game, and can change types for every boss battle after learning revelation dance.
Marowak also technically has low stats but it's useful because thick club.
source
This thing might also be helpful.
