A good pokemon which doesn't disappoint but it's frequently looked down upon is Emolga. Nuzzle + Energy ball is a excellent combo. While acrobatics deals a huge chunk of damage as well. Its speed also allows it to go far in the game, I used one to beat Ghetsis and Alder, for instance. Too bad it doesn't learn Fly.
The elemental monkeys (Pansear, Panpour and Pansage) are also quite helpful if you teach it good moves and reach their peak os usefulness at Lvl 40-50 (if you evolve them), but by the late-game they become weaker and slower compared to the fully-evolved pokémon that normal trainers start to use.
A guts Raticate with toxic orb is also quite powerful, but the problem with this one is that you need investment, so you can't get one of these at your time of need. And when you have the resources ($ and time), you will have much better pokémon available.