S/o to Hex and the crew for coming up with the typing pros and cons. Let's take a look at what's currently on the table.
Water/Steel: Time to jump on the bandwagon here- this typing does pretty much everything we want it to do. Considering the top 2 CM users in OU (Mega Sableye and MG Clefable) are immune to Toxic, it stands as a pretty good guideline for what we want a CM user to have. The Steel-typing also allows this mon to resist T-Flame's Brave Bird and check (perhaps counter) all variants of Clefable, while the Water-typing gives us a fighting chance against Lando-T. The main problem here is a weakness to Fighting, as Mach Punch users can beat DD variants of this mon while Keldeo's Secret Sword laughs at our CM set. The remainder of its flaws can be patched up along the process.
Water/Poison: In all honesty, this appears to be a slightly inferior Water/Steel. While it does shed the Fighting weakness, we no longer resist T-Flame's Brave Bird (although we still threaten it offensively, it can pick us off more easily than it should). Perhaps the biggest flaw is one that Imanalt or nyttyn (I don't remember who it was) brought up on IRC: the Poison typing leaves us weak to Psyshock, which is a big no-no on a CM set.
Electric/Fairy: Fusion Bolt being put off the table places a huge dagger in this typing's viability because of the lack of strong physical Electric-type STAB attacks. While the paralysis immunity is great for the DD set, it doesn't make as much of a difference on the CM set, as opposed to a Toxic immunity, which benefits both sets. We also lose out on the ability to threaten defensive mons such as Lando-T, Clefable, and, to a lesser extent, Heatran.
Fairy/Ghost: Good news: perfect neutral coverage means we can take on most defensive mons with enough boosts. Bad news: we are not resistant to priority attacks outside of Mach Punch, there are no viable physical Ghost moves, and we have no status immunities. Yeah... outside of the perfect neutral coverage, I find that this typing does little for the concept.
Steel/Fairy: It's a buffed version of Electric/Fairy. The Toxic immunity helps both the CM and DD sets immensely, and the ability to get by Clefable allows us to laugh at even Unaware variants of it. However, we still lack a way to get past Heatran, which is one of the main disadvantages of Steel/Fairy over Water/Steel. Nonetheless, Fairy is so stacked on both the offensive and defensive sides of the spectrum that it offsets such a weakness. Think about it: we're not weak to Fighting, we resist Sucker Punch to go along with our resistance to Brave Bird, and we can get past Mega Sableye.
Water/Electric: No Toxic immunity, poor Electric physical STAB moves without Fusion Bolt. Inferior Water/Steel.Also Krilowatt
Poison/Dark: At first I was not a fan of this typing, but after going over it with Imanalt and nyttyn, Poison/Dark is surprisingly pro-concept. The Toxic immunity and the single weakness are crucial for our CM sets. Though I was worried that this typing makes our CM set lose to Lando-T (I shared this worry with Steel/Fairy and Electric/Fairy as well), Imanalt informed me that we don't necessarily need to beat Lando-T in order for this mon's CM set to work. However, we do have some neutral matchups with defensive mons such as Heatran. On #cap, we discussed that neutral matchups may pidgeonhole us into inflating our stats in order to get the upper hand. Though we hit Ferrothorn neutrally with Knock Off on the DD set, the threat of Iron Barbs is there if we can't hit it hard enough. Overall I'm not 100% sure about Poison/Dark but I think it could work.
TL;DR version: My preferences are Water/Steel, Steel/Fairy, and Poison/Dark (not necessarily in that order).
Water/Steel: Time to jump on the bandwagon here- this typing does pretty much everything we want it to do. Considering the top 2 CM users in OU (Mega Sableye and MG Clefable) are immune to Toxic, it stands as a pretty good guideline for what we want a CM user to have. The Steel-typing also allows this mon to resist T-Flame's Brave Bird and check (perhaps counter) all variants of Clefable, while the Water-typing gives us a fighting chance against Lando-T. The main problem here is a weakness to Fighting, as Mach Punch users can beat DD variants of this mon while Keldeo's Secret Sword laughs at our CM set. The remainder of its flaws can be patched up along the process.
Water/Poison: In all honesty, this appears to be a slightly inferior Water/Steel. While it does shed the Fighting weakness, we no longer resist T-Flame's Brave Bird (although we still threaten it offensively, it can pick us off more easily than it should). Perhaps the biggest flaw is one that Imanalt or nyttyn (I don't remember who it was) brought up on IRC: the Poison typing leaves us weak to Psyshock, which is a big no-no on a CM set.
Electric/Fairy: Fusion Bolt being put off the table places a huge dagger in this typing's viability because of the lack of strong physical Electric-type STAB attacks. While the paralysis immunity is great for the DD set, it doesn't make as much of a difference on the CM set, as opposed to a Toxic immunity, which benefits both sets. We also lose out on the ability to threaten defensive mons such as Lando-T, Clefable, and, to a lesser extent, Heatran.
Fairy/Ghost: Good news: perfect neutral coverage means we can take on most defensive mons with enough boosts. Bad news: we are not resistant to priority attacks outside of Mach Punch, there are no viable physical Ghost moves, and we have no status immunities. Yeah... outside of the perfect neutral coverage, I find that this typing does little for the concept.
Steel/Fairy: It's a buffed version of Electric/Fairy. The Toxic immunity helps both the CM and DD sets immensely, and the ability to get by Clefable allows us to laugh at even Unaware variants of it. However, we still lack a way to get past Heatran, which is one of the main disadvantages of Steel/Fairy over Water/Steel. Nonetheless, Fairy is so stacked on both the offensive and defensive sides of the spectrum that it offsets such a weakness. Think about it: we're not weak to Fighting, we resist Sucker Punch to go along with our resistance to Brave Bird, and we can get past Mega Sableye.
Water/Electric: No Toxic immunity, poor Electric physical STAB moves without Fusion Bolt. Inferior Water/Steel.
Poison/Dark: At first I was not a fan of this typing, but after going over it with Imanalt and nyttyn, Poison/Dark is surprisingly pro-concept. The Toxic immunity and the single weakness are crucial for our CM sets. Though I was worried that this typing makes our CM set lose to Lando-T (I shared this worry with Steel/Fairy and Electric/Fairy as well), Imanalt informed me that we don't necessarily need to beat Lando-T in order for this mon's CM set to work. However, we do have some neutral matchups with defensive mons such as Heatran. On #cap, we discussed that neutral matchups may pidgeonhole us into inflating our stats in order to get the upper hand. Though we hit Ferrothorn neutrally with Knock Off on the DD set, the threat of Iron Barbs is there if we can't hit it hard enough. Overall I'm not 100% sure about Poison/Dark but I think it could work.
TL;DR version: My preferences are Water/Steel, Steel/Fairy, and Poison/Dark (not necessarily in that order).