In your answer, please include every tier, as well as doubles format.
Here's the ones I think are the best in their respective tiers:
Ubers: Blissey, Chansey
OU: Chansey, Clefable
UU: Celebi, Blissey
NU: Vaporeon, Mega Audino
PU and below: Audino
1/9/18 Edit: Misread the question and edited this answer, sorry