Well, it depends what you're using it for. For ingame, balance is pretty nice.
Competitively, concentration of stats is usually the best road. Slowbro excels in the Defense stat, but it is knocked out pretty easily by SE Special moves. Slowking, from my experience, is also mix the defenses. As Slowking can withstand more SAtk hits than Slowbro, I find that mixed works wonders when faced with Choice Pokemon. With Slowking's ability to phaze with Dragon Tail, you can easily scout the opponent's team.
- Slowbro: Defensive
- Slowking: Either Specially Defensive or Mixed