Golduck Skillset Problem

Both Psychic and Ice beam are good movesets for Defending Golduck. Personally i would give Ice Beam a little priority, since it's nicely damage grass. If quick attack on your Golduck is Confusion, you're fine with Psychic. If you get watergun, then i would recommend not power it up as it's quite poor defender regardless of it's special.

Keep in mind, though, that CP of Golduck isn't that great in it's maximum, so i't not that great defender in general. If gyms in your area not yet come to 2200+ you can still use it for some time. But eventually CP of Golduck would become serious limitation.

Hydropump does more damage in a neutral type matchup when defending thanks to stab and higher defending DPS
BUT
You will very often face a Dragonite or Plant to kill your defending Golduck. Against who water is at 0,8x damage and ice is 1,25% psy x1,25 against poison (Venasaur, Victreebel, Vileplume).
Plus hydropump is easier to dodge than psychic and Ice beam.

So Ice beam and Psychic are superior to hydropump overall.

Then, given that Psy and IB have same defending DPS (and both no STAB with golduck), I prefer Icebeam because it's effective against more usual attackers while psychic is reisted by exeggutor than can be a good choice for attacking Golduck.

SO IB>Psychic>HP
Confusion is obviously mandatory for defending Golduck.

What's the moveset of your golduck?

I've just fully powered up a 98% Golduck Confusion/IB. Im lvl 33 and he is now 2193 PC. Im happy with it. It wont be as strong as Tier1/1,15 one but still annoying

You shouldn't be calculating "Damage per energy used" as that stat is always far from reality. Also, Golduck is not a very strong defending Pokémon.

When defending, 2 seconds is added between every fast attack (no added delay for charge moves). Theoretically all three charge moves are extremely close to each other (all within 2,5% of each other). Because of this, it is not easy to tell which is the best move for defending.

Ice Beam is easiest to dodge of the three, but like you said it is helpful against attacking grass types. For example if Exeggutor is attacking you Ice Beam should then be the best move, as both other moves are resisted by Exeggutor. For that reason I'd agree that Ice Beam is the strongest of the charge moves. However, Psychic also hits extremely hard and I would rate it above Hydro Pump.