I believe most of the secondary damage is done by the bulkier Pokemon in the OU metagame (Dusknoir, Celebi, Gliscor, Suicune, Vaporeon, Swampert, Bronzong, Cresselia... I can go on forever here), so we need to find types that can deal damage to all them. I've gone over the types and listed all the Pokemon that they hit SE (Note that only Fighting, Ground and Poison are here 'cause they're the only choices in the poll).
These are the Pokemon that Dark currently hits for super effective damage
Celebi, Cresselia, Dusknoir, Latias, The Rotoms, Alakazam, Gengar, Starmie
These ones below are the additional Pokemon that get hit SE for each secondary type
Dark/Fighting: Blissey, Empoleon, Snorlax, Umbreon, Smeargle
Dark/Poison: Pyroak, Breloom
Dark/Ground: Empoleon, Jirachi, Tentacruel, Fidgit, Kitsunoh, Metagross, Bronzong*, Skamory*
*These guys only get hit by Ground attacks when Gravity is active
As you can see, Dark/Ground STABs hit more of the current metagame than Dark/Fighting, even if Gravity is not present. This fact, along with the T-Wave and Sandstorm immunity, is why I'm casting my vote for Ground as the secondary type of choice
Minor Nitpick: Other CAPs should NEVER be part of any analysis of CAP9 because our playtesting process inherently exlcudes them as to get the best test results for the standard OU Metagame. Which removes Fidgit and Kitsunoh from that listing. Fighting is also notable for hitting Skarmory twice as hard when it uses Roost.
Also, shouldn't Heatran be on both of those lists? SR/RoarTran isn't totally out there.














