Rotom-W

[mispy]

Because Tyranitar and Ninetales will likely run rampant to stop Politoed's rain. Honestly, Politoed will be one of your opponent's main targets to kill. It never hurts to have a backup Rain Dancer if Politoed goes down.

Drizzle and Swift Swim on the same team is currently banned, too.

 

[megados]

Just a note, Rotom-W is now the best counter to Gyarados that exists, resisting both STABs, being immune to Earthquake, and sporting neutrality to Payback and Stone Edge thanks to its typing.

I must disagree porygon2 with trace and eviolite can come in intimidate and thunder/thunderbolt it to death with much ease