Off top of my head, one Pokémon I would definitely recommend having on your team is Breloom.
Breloom is rather speedy; it's able to outrun almost any Pokémon opponents send. On top of this, its Attack stat is also good, and it has access to some decent moves as well. For its move-set, I would suggest having something along the lines of Spore, Focus Punch, Seed Bomb, and perhaps Leech Seed. If it has Poison Heal ability, I believe it would be wise if we had given it a set of Toxic Orb.
If I'm not mistaken, Shroomish is pretty easy to get a hold of, as it could be found throughout a lot of places throughout Hoenn (with one being Petalburg Woods). Once we catch it, all we'd have to do is train until it evolves. Aside from that, few other Pokémon I would recommend trying are Metagross (preferably its mega-form), Altaria, Flygon, and Swampert. I hope this helps; good luck, buddy!