Weavile with Night Slash. It is a Dark-type Pokémon that specializes in surprise ambushing, slashing, and it does evolve from Sneasel if holding a Razor Claw during the night.
Marshtomp with Mud Shot. It's the one Ground-type move that you'd be using for the whole game with it, prior to Gen V, at least. Earthquake is a rare or one-off TM that's best reserved either for another physical powerhouse or until you exploit that cloning glitch in Emerald.
Lucario with Aura Sphere. That move was definitely made in mind for it. It is a common association, as Lucario is an intelligent Pokémon.
Volcarona with Quiver Dance. It is the most viable user of such a great setup move before it starts burning and buzzing everything.
Crobat with Confuse Ray. It is a dirty trick that evil team leaders love using sometimes. I dread facing Maxie in particular in Emerald because of how it outspeeds every Pokémon that isn't Electrode.
Snorlax with Rest. Perfect for a notoriously lazy, yet powerful Pokémon back in the day. It's especially good or bad if it has Curse or Amnesia to survive powerful blows.