thundurus + mana
The problem I have with the elec + water cores is that they all feel more like two individual threats. Regardless, I knew I would vote for one, and this is the most difficult to take down. I've never really struggled with Azu, Manec, or the two together, and Thundurus is imo the most annoying elec to face, even though its certainly beatable if you outpredict your opponent. CM Manaphy, on the other hand, can be an absolute nuisance to take down. Its one of the few situations where I'd rather have perish song on politoed, as encore just isn't useful enough to take mana down (at least not without taking a ton of damage from rain and cm boosted scalds). Thund works ok with mana because it pressures torn and can get that twave off on either kingdra or kabu, which eases up manaphy's set-up and sweep a great deal.
Ferro + Gastro
A very annoying combination to take out, this one actually requires toxic on toad, or a ton of pressure from scizor and torn, neither of whom want to take on gastrodon's rain-boosted scalds, and torn wanting to avoid a t-wave from ferrothorn. Overall, I think this is a very efficient defensive core that the team struggles with, and can also get swept by since it struggles to kill curse gastrodon.