According to our gamepress battle sim, the highest DPS against Blissey in the game is...

Highest DPS:

1. Machamp, Counter/Dynamic Punch: DPS =15.14

2. Heracross, Counter/Close Comb: DPS = 12.56

3. Mewtwo, Confusion/Focus Blast: DPS = 12.56

Highest Ability to Win:

0. Ho-oh > Mewtwo > Lugia: Metric = 49.67, 48.64, 40.26

1. Tyranitar, Bite/Crunch: Metric = 39.71

2. Entei

3. Vaporeon, Water Gun/Hydro Pump: Metric = 32.25

4. Blissey, Pound/Hyper Beam: Metric = 31.87

You can see the full results here: https://docs.google.com/spreadsheets/d/1i81N_oU6e0f0z4UWzZN5PXB9TNDJdfPEzd99OoWmYgI/edit?usp=sharing

This includes the new moves, energy costs, and powers from the GAME MASTER file. However, if there have been any fundamental changes in actual gym mechanics, this does not reflect them (since they're not known ;P)