Please advice what's the best moveset for Blissey for defending gym? Thanks.

I noticed huge changes in gym mechanics. I think it will be some time until we know for sure. I am not spending any dust on any Gen 2 evolved forms until we know for sure

IMO its stil the same with Chansey (Pound/Hyper) bc after Chansey evolved her type stil be Normal and just her stats is inscreased. Quick move is Pound for earning energy for Hyper.
P/S: How "delightful" when Niantic changed pokemons CP (nerf and boot) (TwT) || (^w^)

Moves have new damages though, and all said an done given DPS and STAB for gym AI (relatively speaking, we shall get more refined numbers soon) ZH and DG have the highest overall damage output for defense. I am very pleased with mine and hope my hypothesis will be vindicated soon.
PSY - 100
HB - 150 * 1.2 = 180
DG - 2 * 100 = 200
We'll see how cooldown plays in soon.

My Blissey 2327Cp

Pound Hyper Beam :) interesting

Blissey nerf inc. I just tried my Dragonite db/dc cp 3441 vs my Blissey cp 2866 p/dg. I almost ran out of time, I won at 10s left. DG is hit very hard.

I just went up against an over 2900 Blissey with Dazzling Gleam and it was absolutely devastating. It wrecked a maxed Rhydon and Dragonite before nearly timing out.

My money is on dazzling gleam being the best. Hyperbeam has always been easy enough to dodge in the past at least.