[Overview]

There are a lot of baby waters, but Goldeen stands out for its good Attack and Speed and unique access to Lightningrod
Thanks to Lightningrod and Water-type it can hard counter Chinchou, Magnemite, and Return-less Ponyta
Megahorn allows Goldeen to deal heavy damage to Grass-types
Goldeen also gets Swift Swim, but sets like that aren't very good and Goldeen is weak compared to most other swimmer options
The popularity of Scarf Snover makes rain teams pretty weak
This makes it difficult for Goldeen to be useful as a Rain Dance sweeper, but it still has a niche as an Eviolite tank even when Snover is present on the opposing team
[SET]
name: Bulky Attacker
move 1: Aqua Tail
move 2: Megahorn
move 3: Drill Run
move 4: Knock Off
item: Eviolite
ability: Lightningrod
nature: Adamant / Jolly
evs: 76 HP / 220 Atk / 36 Def / 172 Spe

[SET COMMENTS]

Goldeen's good Attack and Speed and usable bulk combined with Lightningrod allow it to serve as a niche counter to some Pokemon that can still deal significant damage to the opposing team
Adamant is important for power, but Jolly is an option to ensure Goldeen outspeeds Scraggy
Lightningrod combined with a pure Water-type allows Goldeen to hard counter Magnemite, Chinchou, and Wild Charge Ponyta
Aqua Tail is used over Waterfall because it gets some important OHKOs and 2HKOs (such as Scraggy and Murkrow), but if you can provide a lot of hazards support and residual damage and hate imperfect accuracy, Waterfall is an option
Megahorn complements Aqua Tail well and allows Goldeen to hit Grass-types hard, OHKOing Snover and 2HKOing Lileep
Drill Run hits the Pokemon Goldeen hard counters -- Magnemite and Chinchou -- supereffectively, as well as 2HKOing Croagunk and Tentacool.
Knock Off is excellent utility to remove Eviolites. Goldeen tends to lure different Pokemon than Mienfoo -- specifically Grass-types like Lileep -- so it can remove their Eviolites to support different Pokemon
[ADDITIONAL COMMENTS]

Works well with teammates like Murkrow that appreciate its ability to shut down Magnemite and Chinchou
Goldeen tends to lure in Grass-types like Lileep or Foongus, so Pokemon that appreciate those Pokemon having their Eviolites removed by Knock Off make for good teammates. Tirtouga likes having a partner that can switch into Chinchou and appreciates having Lileep and Foongus's Eviolites removed so it can OHKO them after a Shell Smash. Goldeen also helps Tirtouga by luring and 2HKOing Croagunk with Drill Run.
Although it can OHKO Snover with Megahorn on the switch (or use Knock Off to remove its Scarf or Eviolite and proceed to OHKO with Megahorn), Goldeen is still vulnerable to being revenge killed by Scarf Snover and thus appreciates teammates that can switch into Grass-type attacks like Ponyta or Murkrow
Frillish and Mantyke hard-wall this set; carrying a Lileep or even a Magnemite or Chinchou of your own can help deal with this
Goldeen cannot effectively stop setup sweepers such as Scraggy or Axew. Riolu counters Scraggy and can Copycat Axew's Outrage to revenge kill it.
Goldeen needs hazards support / residual damage to get a lot of OHKOes and 2HKOes. Ferroseed can provide Stealth Rock and/or Spikes, Snover can provide Hail residual damage and check Drilbur, and Hippopotas can provide both Stealth Rock and Sandstorm for residual damage.
[hide=swift swimmer removed][SET]
name: Swift Swim
move 1: Rain Dance
move 2: Waterfall
move 3: Megahorn
move 4: Drill Run
item: Life Orb
ability: Swift Swim
nature: Adamant
evs: 220 Atk / 36 Def / 36 SpD / 196 Spe

[SET COMMENTS]

Goldeen has a niche among other Swift Swimmers thanks to its access to Megahorn and Drill Run to deal heavy damage to Grass-types with the former and Croagunk and Chinchou with the latter
Hits 30 Speed in the Rain, outspeeding every scarfer slower than Elekid
Life Orb is important to turn 3HKOes into 2HKOes and 2HKOes into OHKOes
Drill Run is used over Return because Return doesn't hit anything notable that Goldeen's other moves can't cover other than Mantyke
[ADDITIONAL COMMENTS]

Stealth Rock and Spikes support helps a lot to give Goldeen a shot at KOing Lileep with Megahorn and Croagunk with Drill Run
Goldeen's sweeps are stopped cold by Scarf Snover, although Goldeen outspeeds non-Scarf Snover even outside of the rain and can OHKO it with Megahorn
A partner like Pursuit Pawniard can remove Snover and give Goldeen another shot at sweeping
Pawniard can also come in freely and set up on most Grass-types like Lileep and Ferroseed
Goldeen hates priority, especially Murkrow's Sucker Punch; Pawniard walls SubRoost Murkrow, and Rain helps it take LO Heat Wave from MixKrow
Tirtouga also stops Krow, likes the boost from rain, and can sweep once Goldeen has weakened Grass-types and Croagunk
[/hide]


[Other Options]

Swift Swim isn't good in this meta and there are better Swift Swim sweepers
Goldeen has a decent special movepool but terrible Special Attack
Toxic, Aqua Ring, and Protect are other support options but Goldeen isn't bulky enough to stay in for that many turns
A defensive set with Scald / Toxic / Knock off can be used, but Goldeen's Special Attack is abysmal and the set would be setup fodder
[Checks and Counters]

Foongus isn't 2HKOed by anything and Spores or Giga Drains
Snover stops rain and revenge kills, although it needs Scarf to avoid getting outsped and OHKOed by Megahorn
Frillish pretty much hard walls Goldeen and burns it to cripple it, though Goldeen can at least use Knock Off to get rid of its Eviolite for other Pokemon to break through it
Ferroseed takes Goldeen's attacks and hits back with Bullet Seed
Croagunk switches into most of Goldeen's stuff and stalls rain with Fake Out, but it hates Drill Run
Goldeen isn't that powerful or bulky, so decently physically bulky Pokemon will probably be able to take its hits and 2HKO it -- although Goldeen will remove their Eviolites with Knock Off in the meantime