Obviously you don't want to give it physical moves so you're left with no choice but to make it forget its powerful Swords of Justice move Sacred Sword. Fortunately, you can replace it by its own unique signature move Secret Sword which also happens to be the best special Fighting move it can learn. The single best move Keldeo can have is Surf, not only is it the best Water move ever but it's also a special move.
Keldeo fears the types Grass, Electric, Flying and Psychic so it's a good idea to teach it moves that are super effective against them. Electric is only weak to Ground and Keldeo unfortunately can't learn a single Ground move. Both Grass and Psychic are weak to Bug and while the only Bug move Keldeo can learn is powerful, it's a physical one. Both Grass and Flying are weak to Ice but the only offensive Ice move Keldeo can learn is Icy Wind and it sucks. Grass is also weak to Fire and Poison but Keldeo cannot learn a single Fire move and the only offensive Poison move it can have is good but physical. Flying is also weak to Electric and Rock but Keldeo cannot learn a single Electric move and the only Rock move it can learn is inaccurate. Finally, Psychic is also weak to Dark and Ghost but Keldeo cannot learn them. It's pretty much screwed against these four types unless you're lucky with Hidden Power.
If you can't cover the types it's weak to, you can at least try covering the types that are weak to Water and Fighting. It can easily beat down Normal, Fire, Ground, Rock, Steel, Ice and Dark types. Unfortunately, it cannot learn a decent move able to cover any of the remaining ten types. The only thing left to do is to teach it non offensive moves. The best candidates would be Calm Mind, Double Team, Reflect, Psych Up and Swagger.
To summarize, Surf and Secret Sword are mandatory and should be its only offensive moves unless you're lucky with Hidden Power. Only teach it Hidden Power if it has a power of 70 and is of the type Bug or Ice since they both cover two types Keldeo fears. Depending on whether or not you have Hidden Power, there are one or two spots left. Choose the move(s) you prefer among Calm Mind, Double Team, Reflect, Psych Up and Swagger. --- I'm French speaking. Male, 22, Quebec