Rain is shaping up to be incredibly powerful, maybe even the most powerful weather this generation. Lots of great offensive and defensive Pokemon to choose from. I don't think Sun can hold a candle to Rain (literally and figuratively).
Actually, with my Sun team I rarely lose to Rain, though very HO teams can cause issues.
Chlorophyll sweepers can absorb T-Spikes with ease and OHKO most water types with their STAB (EQ for Tentacruel on many too). Politoed cannot switch into them even without Sun as they can easily be EVed to outspeed him. Growth sweepers essentially rip through Rain Stall if not played exceptionally well.
As for HO rain, if they do not set-up SR, Tales can simply keep coming in after a sac to turn rain off and cripple their sweepers, and gradually one by one take out the rain team. Granted if they manage to take out Tales you're incredibly screwed, but this rarely seems to occur.
Since Sun teams rely on Grass types to do a lot of their work, I'd argue that they in fact have the type advantage over rain teams, who more commonly use mono-water sets easily walled in sun.
So I decided to look up various ghosts who could help... and I ran accross Spiritomb. Granted he isn't OU in gen4, and he definitely does have his drawbacks, but he is still SO GOOD. A choiceband adament spiritomb with suckerpunch will ALWAYS ohko Starmie. A defensive lead spiritomb can do 70% to 252hp starmie with suckerpunch, and can finish it off with a shadowsneak.
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Also, I forgot about Lanturn! Lanturn is a good raindance team pokemon as it absorbs incoming electric attacks aimed at your other water buddies. Add that to the fact he can be a decent wall (at least in UU) and you have another rain stall pokemon. It doesnt abuse rain dance itself, but it definitely doesnt mind the use of a rain boosted surf.
Also, Mamoswine. He gets his weakness to fire lowered (granted his weakness to water raises), and has the ice typing to be able to take out grass types who pose a threat to raindance teams.
Spiritomb is very interesting, but I think I may try CB Shadow Sneak to 2HKO rather than OHKO with CB Sucker Punch which is total setup bait. He should survive a surf with some investment too.
Lanturn is pretty awesome as a special sponge, but competes with Vappy for a slot. Mamoswine is good but in general I find Natt/Forry/Para take care of opposing grasses excellently while spiking/subseeding.
Has anyone thought of using Tentacruel, not in a rain team, but as a counter?
Rain Dish Tenta can wear opponents down while throwing around TSpikes. Alternatively, Liquid Ooze Tentacruel laughs at Ludicolo and Parasect while flinging STAB SE Sludge Bomb/Acid Bombs.
Enemy Tentacruels can be a pain for Rain stall at least, since they generally lack EQ. Nattorei can use it as reasonable setup bait if they lack spin, and if they lack venom shock then parasect walls it totally. Essentially it can be worn down with repeated surfs and Leech Seed(if Rain Dish)/hazard damage, but it is a pain if significantly bulky.
EDIT: Ran calcs on Spiritomb. Disregarded Sucker Punch due to how horrid it is to be locked into. Discovered that due to Shadow Sneak's low power, you cannot run any EV spread that will effectively beat both LO Spin Starmie and lefties spin Starmie - you only beat one or the other.
The reason for this is that to survive LO Timid Hydro Pump in rain after SR almost all the time you need a 252/252 specially defensive spread. This takes your attack down to a level where you can only 3HKO 252HP starmie. Granted it can only 3HKO you, but they are likely to catch on and switch out before this. The same applies for the 2HKO to offensive Starmie.
Basically the best set to OHKO starmie is a weird one. Specs Modest 252/252 HP/Spdef with Dark Pulse is guaranteed to OHKO offensive Starmie, leaving you at very low health. It has around a 2/3 chance to OHKO 252 HP Starmie, and you are only 3HKOd by its surf.
The issue is coming in on LO Starmies Thunderbolt from a water type it switched into. Running protect on your waters can help you scout the Starmie's intentions somewhat (and lefties since hazards will be down).
Hope this helps someone lol.