I feel like there's a lot of people that are currently looking at superborked stuff in specific typings, but why not discuss something that
works well with lots of types for a change?
tfw when you need to find different art than a different post about keldeo you made before
Keldeo feels a wee bit disrespected without as many appearances as he does in some other formats, where he tends to generally start nuking things off the face of the planet. In Typemons, he just sorta... doesn't exist. People love going ahead and using other threats, most notably that I've seen Greninja, as an offensive mon (in Greninja's case, an offensive Water-type), or they are running semi-stall or even full-on stall, which Keldeo doesn't really fit into. But hey, he actually does have some things that go for him!
...Since I apparently need to make this clear, I'm not saying Keldeo is better or a permanent replacement for Greninja. Keldeo just serves its own purpose over Gren on some teams, and shouldn't be ignored as being 'inferior'.
This statement here could completely go obsolete soon enough, but on Bug teams Tail Glow Keldeo is something to be feared. It doesn't hit nearly as hard as Ashgren does, and without just as much coverage, but not only do its dual STABs hit just about everything at +3 hard enough (Chansey can't wall due to Secret Sword), but it has instantaneous access to its Special Attack, whereas Greninja has to get a kill, usually with a Tail Glow boost. Heal Order's a way, either with Tail Glow or, even better, Calm Mind, to restore HP and give Keldeo a proper method of healing, allowing him to run soloattack sets with Scald, SubCM, and Heal Order, or just a setup, healing, and dual STAB. U-Turn is also a nice option, since pivoting out against walls that hard beat Keldeo ex. Toxapex and fat Grasses such as Tangrowth and Venusaur-Mega into other threats is always nice.
Dark also gives a few nice things, albeit being a niche and probably bad typing that only time will tell. Nasty Plot is a beggar's Tail Glow, and is definitely a worse option but lets you use Dark's wide supporting movepool with the rest of the team. Switcheroo can be run alongside Specs or Scarfed sets in order to throw off defensive teams, and instantly cripple walls. Against Venusaur Switcheroo won't work, so you don't really get any kudos there. Parting Shot is similar to U-Turn, except it can also be run with a Z-Crystal to give free healing to another ally, and can work as a surprise factor if your typing hasn't been revealed just yet or if you haven't revealed what mon is running Parting Shot. Memento is a poor man's Parting Shot, best only if you absolutely need to put Parting Shot onto something else, such as a Regenerator mon.
Ghost is also a type where Keldeo particularly can excel at. Z-Trick or Treat makes it impossible for Keldeo to hit a target with Secret Sword or Focus Blast, but gives +1 to every stat, turning uninvested Keldeo's average 91/90/90 bulk into the equivalent of an uninvested 91/145/145 Pokemon, which is very nice. Moongeist Beam forms perfect type coverage alongside Secret Sword, removing the need for any other attacks if hitting as many Pokemon super-effectively isn't your gist. However, Moongeist is best saved for proper Ghost-types like Gengar or Chandelure who can abuse its power, so Shadow Ball is a better option for Keldeo for the team. Z-Spite can be used as a poor man's form of healing, but it's best to not use that over Z-Trick or Treat for your Ghost-type Z-Crystal.
Then, there's a few types that help grind out its type coverage against specific types. Thunderbolt hits Flying-types that Scald or Hydro Pump don't properly hit and bulky Waters (notably Toxapex and the Slows). Ice Beam is nice for Dragons and bulky Grasses, especially Venusaur-Mega, who Keldeo can barely even do any damage to without (Ice Beam won't do much, but it can secure kills on low % Venus), and Dragonite/Salamence, who resist both other STABs. Flamethrower also hits Grasses, but also can hit Bug-types as well as serve as a neutral option for other moves + being fully accurate if running Hydro Pump. Fire Blast is Flamethrower, but hits harder and probably is best left to something that really needs the power. Shadow Ball, as mentioned earlier, gives perfect neutral coverage alongside Fighting alone, but also lets Keldeo hit Ghost-types and bulky Psychic-types for super-effective damage.
Then, there's the bonus STAB options from Water and Fighting teams. Steam Eruption is incredibly nice as a spammable, high-powered Scald with the same chance to burn, although you may want to save it for something such as another Water-type on your team. Water Spout is perfect for Specs and Scarfed sets for dealing massive damage ala Ashgren in STABmons, as well as punishing switchins expecting a weaker Water move or a different typed move, if you haven't revealed your type just yet. Water Shuriken provides Water-typed Priority, although you have to rely on RNG to get good damage in. Vacuum Wave is really the only good Fighting move, which is special priority that Keldeo appreciates nonetheless. Aura Sphere exists, but if you want to hit Specially use Focus Blast, and if you want a fully accurate move just use Secret Sword.
TL;DR: Keldeo's a good mon who benefits from pretty much every type imaginable, and is an underrated threat. You should try picking him sometime.
If I made any mistakes here, just lemme know
preferably not rudely. I'll try to correct them as time goes, but hopefully I got everything right...