I'm going to chime in my thoughts on the most popular of what has been presented so far, hopefully to detail how each is attuned to our concept assessment and concept in general.
First, let me recap some things from concept assessment that I think are crucial to the success of CAP1 from a typing perspective.
- We're not going to be sweeping
- Good neutral attacking coverage is a good thing for the concept
- CAP1 needs a few serious weaknesses that opponents can use to their advantage and will make their playing against CAP1 more predictable
- CAP1 needs solid switch-ins against key opposing threats that pretty much can't touch CAP1. This means we need to address Pokemon, not types.
Ultimately, these come together to tell me that it's ok to have a weakness to a common attacking type so long as it's made up for by resistances and the capacity to check other common threats. Pokemon like Thundurus and Lati@s are tricky to check in the current metagame, but we shouldn't bind ourselves to checking them or something. If we do, then the playtest metagame will simply see less of those Pokemon and more of what beats CAP1; we don't want that, as that would be a repeat of the issues we encountered long ago in Arghonaut's playtest. Instead, we want CAP1's typing to check an entire swath of Pokemon in the metagame, many of which will continue to be used even in spite of CAP1's presence because they're so good and so varied.
#1: Ghost
For this reason, I think that
Ghost is the strongest type that we can pick. Ghost has a key immunity to Fighting and turns around and can hit Psychic-types for super effective damage. Fighting-type Pokemon (Conkeldurr, Scrafty, Virizion, Terrakion, etc.) are everywhere in BW OU, and Psychic-types (Reuniclus, Sigilyph, etc.) have become very popular as well. In this manner, Ghost enables CAP1 to have many relatively safe switch-ins into many common Pokemon and turn around to threaten them. The fact that Ghost also enables CAP1 to be a spinblocker is icing on the cake, as Rapid Spin, when used successfully, is one of the best ways for an opponent to steal the momentum of a match against a Spike-stacking team. As a last note, Ghost also has respectable neutral attacking coverage with the only types resisting it being Normal-, Steel-, and Dark-types.
#2: Steel
Some people have put down Steel, but remember, friends, this is a Dragon's metagame once more. We're no longer playing in DPP where Salamence, Lati@s, and Garchomp are banned. No, now all of these threats are back, and more have taken hold (Hydreigon, Kyurem, etc.). That crucial Dragon-type resistance gives CAP1 great switch-ins into the quintessential Dragon-types of this generation, and also packs handy resistances to a plethora of other things like Psychic, Dark, Grass, Rock, and so forth. The clutch weaknesses to Fire, Fighting, and Ground are a good place to start for the opponent having a predictable response to CAP1. We would need to cover at least one of these weaknesses with the secondary typing or ability, but that's neither here nor there. For a primary typing, this is strong, and would be a great asset to the CAP. Lastly, the neutral attacking coverage for Steel is surprisingly not bad, being resisted only by Fire-, Water, Electric-, and Steel-types.
#3: Flying
Flying is a great type for this CAP as well, though not as great as the above two. Flying looks great on paper for its immunity to Ground-type attacks, Spikes, and Toxic Spikes, but unfortunately lacks a ton of very safe switch-ins. The only key resistances Flying has are to Fighting-type attacks and Ground-type attacks. Unfortunately, these attacks are always supplemented by a coverage move that beats up Flying-types, such as Rock-type attacks (Terrakion / Landorus / Fighting-types). As nice as the typing seems for those handy immunities, it's really lacking in safe switch-ins for CAP1 to take advantage of and gain momentum, and pretty much forces our secondary typing to resist Rock-type attacks (and thus that typing doesn't actually help with new switch-ins). I don't think this is really the best of ideas, but it's certainly far better than other suggestions like Bug or Ice. Flying also picks up excellent neutral coverage in the current metagame, and actually hits for neutral or better against lots of popular 'mons.
#4: Grass
Grass is cool, but not that amazing here. This goes to the complete opposite end of the spectrum for its resistance to Water-, Electric-, Ground-, and Grass-type attacks. Those are a lot of key types, and its weaknesses to Fire- and Ice-type attacks really aren't that bad. The Fire-type weakness isn't crippling considering that most of the Pokemon whose attacks CAP1 resists will at best be packing HPFire (which sucks). Additionally, the Ice-type weakness isn't too bad because almost nothing actually picks up STAB on their Ice Beams. With enough bulk we could easily overcome that. Grass-types gets a solid switch-in to Rotom-W, for instance, as well as to defensive threats like Ferrothorn and Jellicent. Outside of that, though, Grass really doesn't have a lot going for it. It doesn't have a huge swath of Pokemon it can switch into alone, and pretty much sucks offensively with Steel-, Dragon-, Flying-, Fire-, Bug-, Grass-, and Poison-types resisting the STAB. I don't really think this is the way to go for CAP1, but I've listed it because it's still not awful. After Grass, though, there's really nothing that's been discussed so far or exists that I'm particularly drawn to.
Hopefully this post will have given you guys something to work with for typing!