Politoed/Kyogre is mandatory.
The game's 3 best Swift Swim users are Kingdra, Kabutops and Ludicolo, although Seismitoad is also very good in rain this gen. You'll want one of them, at least.
Tornadus and Thundurus make good backup rain setters, thanks to Prankster and their STABs.
Anything with Water Absorb or Dry Skin are fantastic in rain doubles, since you can hit your opponents with Surf and heal your teammates at the same time. Toxicroak, Heliolisk, Jellicent and even Jynx are good choices
Lati@s, Dragonite, Goodra and Noivern are other Dragons for rain because their movepools and abilities suit it well and they have Water resistances.
Volcarona is unorthodox, but can take on the counters of rain teams easily and has Hurricane to abuse.
Manaphy is great in singles, but setup sweepers aren't so hot in Doubles.
