All of them are very good against quite a few Gyms, sport a decent number of coverage moves, and can hold their own in a battle. I used Rillaboom for my playthrough, but any of the three works very well.
It's an amazing glass cannon. It has a gigantic movepool, including things like Liquidation, Throat Chop, Close Combat, Drill Run, and Poison Jab. It's frail, but that doesn't matter when it OHKOs most of the opponents it faces.
Excadrill is also fairly frail yet fast, which is excellent in a game of hyper offense. Its Ground/Steel typing means it gets STAB on amazing attacks like Earthquake, and helps it deal with a few of the Gym more easily.
Arcanine is a very solid Fire type, and can learn moves like Wild Charge and Crunch for coverage. It deals nicely with quite a few Trainers in the game.
Sylveon is probably the best of the Eeveelutions. It gets very solid attacks and a nice pure Fairy typing that allows it to dish out lots of damage.
Roserade is one of the best Grass type attackers in the game. With moves like Sludge Bomb, Shadow Ball, and Energy Ball, it can cover a wide assortment of Trainers and perform solidly in battle.
Hope I helped!