Thought about it, and Haunter is even more awesome than I thought. First of all, being-ghost-type, it can (apparently) possess things and people without even using a move(think of the possibilites!); as well, assuming that this is the real world and not the Pokemon one, I would want to keep it a secret. It can turn invisible at will (according to the show and every game but the RPG, anyway). No-one would even know I had it!
Also, being ghost-type, it cannot be harmed by any physical objects, such as guns, blades, fists, hammers, etc. making it effectively invincible to just about everyone who hasn't played the games, or even then it would take a fire hose or a flamethrower to hurt it at all, let alone faint it, let alone kill it! It's a ghost, what're they gonna do? Keep in mind nobody should even know it exists. It would be the ultimate bodyguard (or assassin if need be).
Plus, it's really obedient and unlike Gengar won't cause trouble just for the hell of it. So, it can possess people and objects, it can't be killed, it can hide perfectly, and it's loyal. Do I win?