- rain team sun counter; Flash Fire absorber, Grass and Bug resistance, phaser (Roar), immunity to Will-O-Wisp and Toxic.
Rotom-C and Celebi
- rain team sand counter; Leaf Storm STAB will be able to OHKO Hippowdon for eliminating sand, but not T-Tar because of Sp. Def boost. Rotom-C is much less predictable than Celebi is (because no one uses Rotom-C in OU except weirdos like me).
- rain team sand counter and sweeper; Choice Specs, Surf, Secret Sword, Icy Wind, HP Ghost. End of story.
- surprisingly good T-Tar counter; Serene Grace + Iron Head will have 60% flinch chance but is also super effective against Rock. Jirachi can carry Thunder, T-Wave or Body Slam for paraflinching, set up Substitute in Sand (no buffeting) or run a Trick set.
Gastrodon (I'll add a sand rain counter so you know what you need to deal with)
- sand team rain counter; Storm Drain, Recover stall, good bulk and typing (one weakness), decent offensive presence.
- T-Tar, Ninetales, Volcarona counter; Choice Scarf, Close Combat, X-Scissor, Earthquake, Stone Edge
(meme stolen from one of Mew's teams)
(but watch out for Gliscor and Skarmory)
Your main goal when facing another team is to eliminate the weather starter while keeping yours still in the game. If you can eliminate Sun/ Sand, the weather boosts will be gone and you can sweep in rain.