RE: What do you think is the best weather force?
The amount of tools rain has right now make it easily the best.
Even if you just scratch the surface, you've got Pokemon like Tornadus-T, Thundurus-T, regular Tornadus, Rotom-W, Starmie, Jirachi and Dragonite. All of these Pokemon are the front liners of rain based offense, and have the speed, coverage and offensive prowess to make teams tumble over.
Then you get into defensive roles and you find none other than Tentacruel and Ferrothorn. With Rain Dish, Tentacruel is INCREDIBLY resilient, especially if it inflicts a Scald burn. Ferrothorn is a bit tricky to use in BW2, but under rain it's also very tough to take down without a Fighting-type.
Then you get other support stuff like Genesect, Scizor and Dugtrio. Genesect works on close to every team, but it's especially solid on rain where it's sole weakness gets halved, and it can go for perfect accuracy Thunders. Scizor can also pick off weakened rain threats like Latios with a Pursuit or Bullet Punch, but it's kinda fallen a little bit due to Genesect existing. Both sun and sand, OU's other top weathers, happen to have a weather setter weak to Ground, meaning Dugtrio can trap them in battle and pick them off.
Heck, most of OU works on rain teams, except for like Ninetales and Tyranitar. Even new suspect Kyurem-B works on rain too.
I think Gastrodon's climb to fame (went from NU to OU in one tier change) is a big testament to the power rain has in the metagame.