Interesting to see this pony getting so much attention...and disappointing to see so much theorymoning rather than people actually playing DW.
Anyway, it will be rather interesting to see how Keldeo will initially play in standard since I'm fairly sure not a lot of standard players know his "counters" (Poliwrath is preeetty~ common, eh?). The quick list would be:
Dragonite
Salamence
Slowbro
Slowking
Jellicent
Celebi
Gyarados
Starmie
Latios
Latias
Toxicroak
Amoongus (Only good in DW)
Alakazam
Serperior (DW)
Tornadus
Whimsicott
Venusaur
Tangrowth
Jolteon
Ditto (DW)
And that's of course without diving into the plethora of Choice Scarf Pokemon that can take him down like Terrakion who doesn't mind revenge-killing his little brother.
But as you can see, there is no correct choice of Hidden Power for Keldeo since he needs more than one type to cover all those weaknesses. Look here:
HP Ghost sets gets stopped by Dragonite, Salamence, Gyarados, Amoongus, Venusaur, & Serperior.
HP Ice means Jellicent, Gyarados, Starmie, & the Slow-Bros get the best of him.
HP Electric leaves him open to Celebi, Latios, Latias, and ...well everything HP Ghost does minus Gyarados.
I personally believe HP Ghost to be the worst of the 3 Hidden Powers because you lose to Gyarados and the Pokemon that you're trying to hit with HP Ghost either:
1. They're not OHKO'd by a +1 HP Ghost (Celebi, Latias, & Latios, although Latios has a % chance of being KO'd)
2. They're faster than Keldeo & KO him first (Latios, Latias, & Starmie)
3. They're faster & can setup on +1 Keldeo's HP Ghost (CM Latios & CM Latias)
Aside from maybe Celebi if it's not running Leaf Storm or Status, you're losing that battle anyway or at most 2HKOing them, (which +1 Rain-boosted Hydro Pump should already do) so you might as well get the extra "practical" coverage HP Electric gives you even if it is just Gyarados since he completely forces Keldeo out with Bounce or Dragon Tail.
But again, that's just my opinion on the matter. There's probably a Pokemon I'm forgetting or never ran into that I'd absolutely need HP Ghost for.