If this is a prefference post, why is Ground winning in such a margin? I really don't know why, as the choices are pretty equal in effectiveness.
Why was Dark chosen in such a landslide victory in the first poll? That's what people chose. Who knows how the minds of others work? I only know how I think, and subsequently, what results of that. No one else.
I'll actually be off for quite a few hours after this, due to IRL issues. However, before I go, I'd like to list something. Below are the most commonly seen Thunderwave users in OU. Actually, they are really the only viable users at all. This is the percentage of them that the move has been seen on.
Blissey | Move | Thunder Wave | 37.3
Celebi | Move | Thunder Wave | 33.3
Cresselia | Move | Thunder Wave | 42.4
Porygon2 | Move | Thunder Wave | 49.0
Togekiss | Move | Thunder Wave | 43.9
So, out of the 5 Pokemon in OU that actually run thunderwave, you threaten two of them. Cresselia will not enjoy STAB Dark anything. However, depending on the Speed of our CAP, it could just Reflect up and laugh it away, (Something to take into consideration if Ground is chosen.) while being immune to our Ground STAB. Blissey doesn't like taking anything physical, so that is a bit of a moot point, imo. Celebi would easily threaten you back out with Grass Knot (Depending on weight) or Leaf Storm. Porygon2 and Togekiss don't really care about either STAB. As a matter of fact, Porygon2 can take advantage of whatever ability it is given on the switch in while Togekiss just enjoys its free switch in to a Ground move.
Of course, this is all a bit of a stretch, as movepool, stats, and the ability all could be used to take care of this. I just wanted to outline some issues Ground may have before leaving.