I think having the higher Special Defence is better for resisting those type, so my answer is Slowking.
The most common Ghost attack is probably Shadow Ball, which is a Special Attack.
Dark has two common attacks - Dark Pulse and Crunch. Crunch is probably more common, simply because many Dark types are physical attackers, and quite a few non-Dark Pokemon also have access to it for coverage. So that doesn't help much when deciding if Defence or Special Defence is more helpful.
The most common Grass attack is probably Giga Drain or Energy Ball. Solar Beam is also often used on Sun Teams. All of these are Special Attacks, so that is pretty easy. The only physical Grass attack you might see is Leaf Blade, but it has quite limited distribution.
As for the Electric type, most things will use Thunderbolt or Thunder. As they are Special Attacks that is another score for Slowking.
Finally the Bug type. X-scissor is probably the most common Bug attack, thanks to its TM status. For those Pokemon that get it, Bug Buzz is also a fairly common move. Like Dark, there is no clear winner here. One common move is Special, the other is Physical. X-scissor is probably more common, since it is a TM, and is a decent coverage move for some physical attackers.
So overall, Electric, Grass, and Ghost usually have Special moves, while Bug and Dark have common moves in both categories. Therefore I think that Special Defence will help more with his weaknesses than Defence.