My favorite keldeo set by far is LO under rain. I ran a team where the whole purpose was to get as many layers of spikes up as possible, get rid of opposing weather, and then have keldeo go to town. Seriously, very few pokemon can take a single hit from keldeo with a couple layers of spikes (and hopefully rocks). I won a ton of games simply by spamming spikes, switching in politoed, letting it die, and sweeping with keldeo.
However, as powerful as keldeo is, I think there is one crucial factor that keeps it very much not-broken; its inability to hurt its counters. This is primarily due to its extremely limited movepool; if this thing got ice beam and/or thunderbolt, it would probably be broken. As it is though, however threatening keldeo may be, it is very easy to throw one of the pokemon that counters it onto the team and never have to worry about it again. For example, I currently run a sand team that is extremely weak to both water and fighting, with a bulky latias thrown onto it, and I can't remember the last time a keldeo got a KO against me. I'm not saying it's not good--just that, despite its power, it really is not too hard to just shut it down.