NU - OU
Goluck
Used for his ability, Cloud Nine.
Licklicky
Also used for his ability, Cloud Nine. He is also the most popular weather stopper.
Altaria
The least used weather counter, once again for Cloud Nine
Ubers
Raquaza
He has Air Lock, and is extremely powerful, anymore to say?
And that's pretty much it, you also have to look out for pretty much any flying type with Defog, although that isn't a popular approach.
And any random pokemon someone decides to use to set up weather to counter other weather.
Example:
Umbreon@Heat Rock
EVs: 252 Def - 252 Hp - 4 Sp Def
Impish Nature -- + Def :- Atk
- Sunny Day
- Moonlight
- Payback
- Curse
You can take tons of random pokemon and throw things like that on them. Heck, Sawk can learn Sunny day. Those are the note able weather counters though.