Modest/Timid, 252 SpAtk, 252 Speed, 4 HP
Drizzle, Life Orb
- Surf / Hydro Pump
- Ice Beam
- Hidden Power [Grass] / Psychic
- Hypnosis / Toxic
Surf and Hydro Pump have huge power, STAB, and Rain boost, which adds up to huge damage after adding in the Life Orb. Ice Beam has incredible coverage, hitting the dragons that Politoed will probably be facing in higher tiers (Lets face it, he's gone up tiers this generation thanks to Drizzle). HP[Grass] catches other bulky grass types, Psychic catches a majority of the new Fighting types and has more base power. Hypnosis is an incredible, if somewhat risky status move, but Toxic is good to have if you aren't carrying HP[Grass] to take out other bulky waters.
Partners (Double / Triple Battles)
Lanturn and Politoed make a great duo with Rain and Thunder, and Dream World Lanturns (Though they aren't that common), can have Water Absorb to pair up with Surf. Almost anything that could use Thunder, should be with Politoed, to take out other bulky waters. Lapras is another good partner for the same reasons as Lanturn. Other than that, you want coverage against bulky grass types, which can be covered with Flying attacks, since Politoed weakens Fire attacks. Crobat with Hypnosis and Air Slash would be an interesting partner, providing extra sleep support and flinching.
Bulky Grass types, most physical attackers, or anything carrying Thunder. I know that this is a broad list, but here's the math. Politoed has 75 Base Defense which isn't very impressive, only carries Ice Beam to deal with Bulky Grass types, and a solid number of them are faster than Politoed, and Thunder is getting 100% accuracy off of his rain. Galvantula has a party with Politoed, since it's probably already carrying Thunder and Compoundeyes, and Energy Ball is an option for it.
This is a signature that doesn't eat up an absurd amount of space.