Notily has my vote.
The other options seem kinda redundant in typing areas and leave the team kinda in an awkward state. For example, Diancie does give the team a flying resist, but Stealth Rocks are kinda mandatory and a reason for using Diancie, and double Fairy is really bad across the board. Nihilego also gives a flying resist, but double Poison types are like the last thing you want due to the prevalence of psychic types like Latias and Celebi, as well as scarf Krook being common still. Empoleon seems a little better then Nihil/Diancie, but the double water types here leave the team really annoyed by the likes of Rotom Heat and Mega Manectric, which both can pressure Nidoqueen with hidden power Ice/Overheat, respectively.
With that all being said, Rotom Heat stands out as the best option given here in my opinion due to offering a Flying Resist, another Scizor check, as well as Volt Switch support to bring in it's teammates, which is an added bonus, and doesn't seem redundant, at least to me. No hard feelings to the other people who submitted the other options, that's just my views on the matter.
tl;dr - I'm voting Rotom-Heat.