I like Electric
now I'm not great at pokemon so my reasons will probably be shaky to experienced people, but whatever
I was looking at the number of weaknesses each of the combinations had. Dragon and Ground both have 4 weaknesses, which is by no means bad, but trying to minimize the amount of moves that can smash it is important to me.
Poison, Poison/Steel, and Poison/Electric all have two weaknesses. I threw out steel because it won't absorb toxic spikes, which is the main reason I wanted poison. Poison/Dark only has one weakness.
Pure poison has 3 resists if I remember correctly.
Poison/Dark has 4 resists and 1 immunity
Poison/Electric has 7 resists
Dark gives up resistances on fighting and bug, but also gets rid of a weakness. Electric doesn't mess with Poison's resists, and brings a few more to the table, while making it more susceptible to ground moves. Electric has a slight advantage in the resist category in my opinion.
I also looked at useful support moves that are of each type. (this isn't as important since a lot of pokemon learn moves that aren't their type, but it's a close race for me)
Poison - Gastro Acid, Toxic Spikes, various poisoning moves
Dark - Dark Void, Embargo, Snatch, Switcheroo, Knock Off, Taunt, Theif, Memento
Electric - Magnet Rise, Thunder Wave
Dark clearly has a lot of support options. Magnet Rise looks nice, but Dark beats out quality with quantity, 4 times as many possibilities
Yet I really want to vote electric. Some of the more important dark support moves (knock off, taunt) are learned by more non-dark pokemon than pokemon that actually are dark-type. And Dark Void isn't learned by anything save darkrai. (I dunno if dark wins, you might give it dark void, but I think I'm getting too far ahead of the poll, and I need to come back to typing).
After all this stuff, I can't really justify that electric is better than dark, or dark is better than ground, or electric is better than no additional type. I think that all of these options will yield a good pokemon with sunday moderating the CAP threads, so I'm just going to vote for electic whatever