Technically, the best shadow-ball user in the game is deoxys, with the highest attack stat in the game when in attack forme.
However, you probably want one that you can get before the Elite 4:
So using showdown to find the highest-attack shadow ballers I got these results (eliminating Pokemon that can't be gotten before the Pokemon league) for Ruby/sapphire:
Slacking is very good, and can easily be found and added to a team, as long as you don't mind truant (let's be honest, you do)
Zangoose has good attack and is fairly balanced
Banette has the same base attack as zangoose but also gets STAB
Exploud is probably the easiest to get, and has decent attack and move-pool.
After the league, you can get Metagross, who has one of the highest attack stats in the game, and is actually has the third-highest attack stat of any Pokemon that gets shadow ball
flareon surprisingly has stellar attack and access to shadow ball, but is a bit lacking in its move-pool
Snorlax is higher defensive prowess, as well as a great move-pool, but is limited by speed; it also gets two useful abilities in thick fat and immunity.
Nidoking is another threatening shadow-ball user, with the added threat of having great typing. shadow ball is also generally useful on nidoking as a cover for psychic types. Nidoran is also fairly easy to find early on in the game.