I know people have done this kind of thing about Golduck already, but I just wanna state my opinion. Golduck are epic all-rounders with an above-average 95 base Special Attack. They've got a pretty deep movepool as well. Here's a few sets that our fine blue friend can use:
Shedinja Support
Nature: Hasty
EV Spread: 240 Speed, 240 SP Defense
Ability: Damp
Held Item: Anything that helps.
Move 1: Soak
Move 2: Dig
Move 3: Surf/Hydro Pump
Move 4: Anything goes.
As the title says, Golduck can be epic on a Shedinja-based team. It's packing Soak in its level-up movepool, which turns the target into a Water-type. This is good because when Soak is used on Shedinja, it'll only have two weaknesses! Plus, it'll be able to survive Surf even without Soak because of Wonder Guard. Dig helps Golduck survive a few turns against things like Raichu and Magnezone. Pick anything you like for the fourth move. It doesn't really matter. It's also got Damp to neutralize common Explosion users like Foretress and Electrode.
Surf's up!
Nature: Modest/Bold/Timid
EV Spread: 252 SP Attack, 100 Speed, 100 SP Defense, 100 Defense
Ability: Swift Swim (Dream World only)
Held Item: Damp Rock/Mystic Water/Water Gem
Move 1: Rain Dance
Move 2: Surf
Move 3: Psyshock
Move 4: Calm Mind
If you got Golduck from the Dream World, it should have Swift Swim. This means its Speed is boosted in the rain. Rain also powers up Water moves by 50%, so you'll be reeling of super speedy and very powerful Surfs, and if you decide to give Golduck the Water Gem or the Mystic Water, it'll get even stronger. Plus, Surf is one of the best STAB Water moves out there for Golduck. Psyshock is great against weak physical defenders like Hitmonlee and Cryogonal, because Psyshock deals physical damage. Calm Mind will up Golduck's SP Attack and SP Defense even more.
Other options
Golduck gets Encore as an egg move, so if anyone is reckless enough to have a Reckless Emboar with a Life Orb use Flare Blitz, Golduck just shrugs it off, and they take massive recoil from Flare Blitz and the Life Orb, which can be even deadlier if Golduck has a Rocky Helmet. They also get Foresight and Cross Chop through breeding. Pair the two moves together, and Spiritomb lovers will wet themselves.
Checks and Counters
If not EV trained in SP Defense and Speed, Golduck will fall pretty easily to an Electro Ball-wielding Galvantula, Raichu or Electrode. They can also take some heavy damage from Adamant Electivires. They're also not the fastest runners in the race, so speedy revenge killers like Choice Scarf Scizor, Hasty Primape, and Leafeon can take it down easily.
