Assuming this is for the singleplayer campaign, the best Pokemon are usually the ones that can deal the most damage. You should be looking for Pokemon with high attack stats, powerful moves, or boosting moves, and a good offensive typing helps too. These dark Pokemon best fit the criteria.
Houndoom @ dread plate/choice specs
Ability: flash fire
EVs: 100 SpA
- flamethrower
- dark pulse
- sucker punch/sludge bomb/rock smash/strength
- sucker punch/sludge bomb/rock smash/strength
Houndour is obtainable a lot earlier than most other dark Pokemon, and it can immediately learn fire blast and evolve after you catch it. Fire dark is a great offensive type combination, but it doesn't learn much outside of its fire and dark moves. It can learn nasty plot before Cynthia only if you put in a lot of experience.
Weavile @ dread plate/never melt ice/expert belt
EVs: 100 Atk
- ice punch
- night slash
- swords dance
- dig
Weavile is another Pokemon with great stats and offensive typing, and it learns swords dance. It evolves later than most other Pokemon, though.
Absol @ dread plate
Ability: doesn't matter, neither are all that great.
EVs: 100 Spe
- night slash
- swords dance
- psycho cut
- sucker punch/X-scissor
Absol has the highest attack stat of any dark Pokemon in the regional Pokedex, and it learns a reliable STAB, a boosting move, and a good coverage move just by leveling up. Its biggest flaw is that it's obtainable later than almost all other Pokemon.
Floatzel @ mystic water
Ability: swift swim
EVs: 100 Atk
- waterfall
- rain dance
- ice punch
- crunch
Floatzel deserves a mention for being probably the best non-dark Pokemon with a dark move. It learns crunch by level up very early in the game, and it's effectively the only good option if you want to use a dark move against Fantina. It also gets real physical STAB, unlike Luxray. Floatzel's attack stats unfortunately aren't the best, so it doesn't always do enough damage even when using waterfall.
