You should get a Gengar imo, try and find a timid/modest one with decent iv's. Gengar is one of my personal favourites and is really strong and versatile because of its speed and ability to take out most types. A would recommend it knowing psychic (get the tm from the psychic gym), thunderbolt (tm from 3rd gym), dark pulse and shadow ball. Having a move like thunderbolt is good for later in the game when you'll be grinding in seafoam against water types and golbat.