First thing I noticed; there's nothing on your team that abuses the two 100% moves in rain, Thunder and Hurricane. Dragonite is the most common user of both, gaining STAB from Huurricane, which will be really useful in order to counter Grass types.
Politoed @ Leftovers
Ability: Drizzle
EVs: 252 HP, 252 Def, 4 Sp. Def
Calm Nature (or Sassy Nature, if you want to ensure lowest speed possible to have Drizzle prevail)
- Scald/ Hydro Pump
- Protect
- Ice Beam
- Hypnosis/ Toxic
I've seen many offensive Politoeds and I feel as if they bring a major drawback to the team. I know that taking up a team slot with a mediocre Pokemon that is only needed for its ability seems to be a bit of a waste, but I would much rather go for defensive on your weather Pokemon, because as weather is so prevalent in OU, if your weather Pokemon faints your team falls apart. And why would you put Hypnosis on a Choice Specs set?
Vaporeon @ Leftovers
Ability: Hydration
EVs: 4 HP, 252 Def, 252 Sp. Def
Calm Nature
- Rest
- Heal Bell
- Ice Beam
- Scald
If you have a rain team planned, Hydration is the superior ability on Vaporeon; it will wake up from Rest immediately. Ice Beam to provide coverage, and Heal Bell to provide team support.
This is the set I run on my rain team Vaporeon and I find it works really well.
Tentacruel @ Black Sludge
Ability: Rain Dish
EVs: 252 Sp. Def, 4 HP, 252 Def
Bold Nature
- Toxic Spikes
- Rapid Spin
- Knock Off
- Scald
OR
Forretress @ Leftovers
Ability: Sturdy
EVs: 252 HP, 252 Def, 4 Sp. Def
Bold Nature
- Stealth rock
- Rapid Spin
- Spikes/ Toxic Spikes
- Volt Switch
I would replace Ferrothorn with either Forretress or Tentacruel; you don't have a rapid spinner. Tentacruel has Rain Dish and Poison typing, which means that Grass is now x1 and it will absorb Toxic spikes. Forretress is steel so it doesn't have to worry about Toxic Spikes.
Gliscor @ Toxic Orb
Ability: Poison Heal
EVs: 252 Atk / 184 HP / 72 Spd
Adamant Nature
- Earthquake
- Swords Dance
- Ice Fang
- Roost/ Protect
Standard set for Gliscor, not much else to say.
Mamoswine @ Choice Scarf
Ability: Thick Fat
EVs: 252 Atk / 252 Spd / 4 HP
Adamant Nature
- Earthquake
- Stone Edge/ Rock Slide
- Ice Shard
- Body Slam/ Ice Fang
Ice Shard over Icicle Spear, it will give a priority move to Mamoswine and Icicle Spear is only worthwhile with Skill Link. Rock Slide is less powerful than Stone Edge, but has more accuracy.
What role does Volcarona fill on your team? Is it meant to be a Sun counter?
Volcarona @ Leftovers
Ability: Flame Body
EVs: 252 Spd / 252 SAtk / 4 HP
Modest Nature
- Bug Buzz
- Quiver Dance
- Flamethrower
- Roost/ Giga Drain
Roost is an option; Giga Drain might not be that useful against sun teams because of the fire and grass types that are usually there to abuse it. You can consider Chandelure in preference to Volcarona; it has Flash Fire but more weaknesses than Volcarona.
Other options you can consider are Toxicroak (Dry Skin in rain heals 1/8 of health) and Dragonite (Hurricane and Thunder user).
