Ghosts are simply the greatest.
Gengar. He is amazing; resistant to fighting, ground and normal moves, is helpful in battle because it sometimes limits what they can use. He can learn hard hitting attacks like thunderbolt and earthquake if i'm not mistaken. His stats are decent as well.
In the anime, all he does is fool around and play jokes which I think is cool for a pokemon since most have the same attitude with minor tweaks. I'm pretty sure he can turn invisble/visible at will and play with your shadows.
