I pick Chimchar.
1.Excluding Ponytas, not many Fire-Type are in Sinnoh.
2.If you train it to an Infernape, you can stand a chance at Empoleon(Steel isn't weak against Fighting) and just KO Torterra.
3.Great KO'er with that Attack stat because it's lots of its move are physical.
4.Speedy in case there's trobule.
5.After evolution, you can KO the 1 and 2 people of the Elitle Four.
6.Sinnoh has a lot of Wild Grass Pokemon, so it's helpful.
5. Even thoguh its hard to level up, you can still train in the Eterna Forest with Cheryl because her Chansey auto heals your Pokemon after a battle there( that's why it's a good place to train any Pokemon).
........................Must I go on?