So after the updates to PvP, a lot more people having been playing PvP and I've been tracking the pokemon in the battles that I've been facing.
After a sample of 60 PvP battles, this is what I have (by % seen in matches):
1) 36.67%: Scizor
2) 25.0%: Azumarill, Starmie
3) 23.33%: Blissey, Togekiss
4) 21.67%: Gyarados
5) 20.00%: Gengar
6) 16.67%: Machamp
7) 15.00%: Tyranitar
8) 13.33%: Ferrothorn, Talonflame, Arcanine, Milotic, Skarmory, Weavile
9) 11.67%: Sableye, Gliscor, Electivire
10) 10%: Snorlax, Dragonite, Magnezone, Donphan
Note: Sample is from Blue Server
My rating is usually near the 200, but from my experience, the ratings of the players I'm against are pretty random (I've seen players from 0-450).
I will continue updating the numbers as I play PvP.