So far LO Mamo, EB Keldeo, and Band Terrakion have been the best submissions, and the only good ones so far. Here are my comments on them:
EB Keldeo:
The combo of Zapdos + Keldeo is countered only by Gastrodon, Amoonguss, and CM Latias, all of which are Pokemon with little immediate offensive presence and not so hard to counter or take advantage of. Keldeo also outspeeds Landorus and resists Hidden Power Ice, making Landorus think twice before firing them to catch Zapdos on the switch. Keldeo is an amazing offensive Pokemon that keeps up excellently the offensive pace and leaves little breathing room to team 1, forcing them to either use Pokemon with little offensive presence or offensive checks (and thus Keldeo will still be able to get past them with prediction), which means that no matter what, Keldeo will be useful.
LO Mamoswine:
The combo of Zapdos + Mamoswine is countered by zero Pokemon in OU, but is checked by much more Pokemon that the Zapdos + Keldeo combo, due to Mamo's lower Speed, which means that Pokemon such as Kyurem-B, offensive Water-types in rain, and Terrakion all have the potential to greatly threaten this core. While it is true that having only checks is better than having counters too, the Pokemon that check this relatively slow combo are very popular and very dangerous, with huge offensive presence, that are difficult to take advantage of, unlike the hard counters to the first combo, which are all defensively inclined Pokemon that are easy to handle. This leads me to believe that Keldeo is a better pick than Mamoswine, even though Mamoswine is great as well. I just find that Keldeo has much better all around utility than Mamoswine, which i believe is a Pokemon that we should pick in later stages if we are desperately in need or its priority or wallbreaking powers, as Mamo's use is much more focused and specific than Keldeo's.
CB Terrakion:
The combo of Zapdos + Terrakion is countered only by physically defensive Hippowdon, and this without SR up, as with SR up even Hippo gets 2HKOed 30% of the time from CC. However, the Hippowdon problem is quite a big one, imo. Hippo beats both Zapdos and Terrakion 1 on 1 (Rock Slide/Ice Fang + EQ while Zapdos Roosts and even Toxic is an option), gets ample switch-in opportunities against Zapdos while also blocking Volt Switch and sets up sand and SR, both of which will hinder Zapdos's ability to deal with Landorus. Of 'course our later picks could make sure that Hippowdon is threatened to a big enough degree to not do as he pleases but this is easier said than done as Hippowdon is an a great all around wall and not so many Pokemon can get past it. So, even though we may pick one-two Pokemon that will be able to deal with it, starting from the get go with two Pokemon that lose 1 on 1 against such a great and prominent threat doesn't seem as the best option to me. Furthermore, i don't really like picking Choiced mons this early, as they can lead to the opponent picking Pokemon that can take advantage of our choice locked moves and thus forcing us to play defensively after, which should be avoided.
tl;dr EB Keldeo > LO Mamo > CB Terrakion > everything else so far