I'm assuming Surf / Mystery Sword / HP Ice / Hydro Pump? Specs Keldeo is a beast in rain. Just another abuser for the ability should Drizzle somehow stick around until it is released.
I would say stick to HP Ghost, since it covers Latios, Latias, Toxicroak, and Celebi all at once. Gyarados and Dragonite can be taken care of with a +1 Hydro Pump in the Rain if you have Stealth Rock up.
Wouldn't HP Ice hit all 4 of those Pokemon with equal damage to HP Ghost? And it would hit Dragonite too, even though you did say Hydro Pump takes care of it.
A single stat point won't matter in anything besides Speed, and Ghost doesn't touch your speed IVs at all.i would actually rather use dark, as he can maintain his perfect IVs. but i guess toxicroak is kinda a problem.
Wouldn't HP Ice hit all 4 of those Pokemon with equal damage to HP Ghost? And it would hit Dragonite too, even though you did say Hydro Pump takes care of it.
The idea is that no other Hidden Power can cover Latios, Latias, Toxicroak, Celebi, Jellicent, Slowbro, and Slowking all at once. Threats that Hidden Power Ice cover are destroyed by boosted rain water attacks after SR.
Have people considered a Specs set for Keldeo at all? Hydro Pump / Sacred Sword / Hidden Power Ghost / Focus Blast gives you the choice between powerful Special and Physical Fighting attacks that might be helpful. Shame it doesn't get Ice Beam, because then it actually could 2HKO the ENTIRE OU metagame.
Dragonite is a pretty good response, even if Multiscale doesn't trigger. Physically Defensive Shaymin would also do nicely.