Golduck is a mid-level UU sweeper with Calm Mind and Hypnosis. None of its stats are particularly bad, and its Speed is actually decent in UU, so it will usually live through a hit or two and dish out some damage. With Cloud Nine, it can be very useful to stop a sweep from Chlorophyll or Swift Swim enemies: thanks to Cloud Nine, it is not affected by the weather, allowing it to Ice Beam that Victreebel or Vileplume, or Surf that Omastar. Damp can be funny to stop Solrock from Exploding on you though.
Calm Mind is your best bet when using Golduck. Just build up your Special Attack with Calm Mind, Hypnotize your opponent's main Golduck counter and try to sweep. Hypnosis and Ice Beam are replaceable and depend on personal preference as well as your team. Psychic can be used as very few things resist both Surf and Psychic, and it hits Vileplume and Muk hard. Hidden Power Grass can be useful as well, it OHKOs Quagsire after a Calm Mind (who cannot be touched by Surf) as well as Omastar, and it hits other Waters as well. Substitute in the last slot makes Golduck a lot like the standard Jynx and can be rather scary as it cannot be walled as easily by Waters with Toxic anymore, but you'll hurt your attacking versatility.
Damp lets you block Explosion, but Cloud Nine lets Golduck switch in on a weakened Sunny Day Chlorophyll Pokémon, force it to charge up Solarbeam, and finish it off with Ice Beam. Of course, doing this also means you cannot benefit from, say, an enemy Omastar's Rain Dance.
Hydro Pump can be used over Surf anywhere for more power. Toxic is nice for other Waters; it will really get in the way for Grumpig and Hypno in UU. Refresh can shrug off status, but you're better off preventing it altogether. Psych Up can snag Calm Minds, but Golduck isn't likely to win a Calm Mind war since its stats are so mediocre. Light Screen is nice for support, but Golduck isn't the best user for it.
Mixed sets need some Attack EVs to hurt the standard Special walls. You don't need more than 220 Attack, though, usually. You generally just want to raise Golduck's Special Attack and Speed as much as possible, as it's meant to sweep.
Checks and Counters
Meganium and Vileplume resist Surf and can use STAB Grass moves, put Golduck to sleep, Leech Seed it, or Toxic it. However, they surely won't like Ice Beam, and Vileplume doesn't like Psychic either. Lanturn makes a great counter with its resistance to Surf and Ice Beam, and STAB Thunderbolt to put an end to Golduck. Hypno has Insomnia, great HP and Special Defense, and will whittle Golduck down with Toxic, Thunder Wave, or Seismic Toss, or even Calm Mind up on Golduck. Grumpig doesn't have Insomnia or Thunder Wave, but will often beat Golduck in a Calm Mind war or Trick a Choice Band onto it. Quagsire can be a prick because it can restore health on Surf and hit Golduck with STAB Earthquake.
Remember, Golduck often packs Hypnosis, so you may need Sleep Talk on your counter, or absorb that Hypnosis with something else before sending in your counter.