My favorite used to be froslass, but it has to be smoochum and jynx now. I really liked froslass because of the mind games it could play as a spikes setter with destiny bond and obviously the design.
The reason why I like smoochum now is because of what happened in pokemon crystal. I was hunting the odd egg and I got a smoochum. I played the rest of my game with the smoochum and it was a really useful member of the team.