Gen 5 OU
Keldeo-R @ Expert Belt
EVs: 252 SAtk / 4 SDef / 252 Spd
Timid Nature (+Spd, -Atk)
- Secret Sword
- Icy Wind
- Hidden Power [Bug]
This specific Keldeo is one of the best checks to Celebi, in my opinion, It just catches so many Pokemon off guard due to the current popularity of the Choice Scarf set, which runs Surf / Hydro Pump / Secret Sword / Hidden Power [Ice]. Easily walled by Celebi, and even an HP Ice does around 40%, which Celebi can simply recover off with Recover or Giga Drain.
Expert Belt is the item of choice, so Keldeo can easily bluff a Choice Scarf / Specs set, causing an easier time dealing with the number 1 switchin into Keldeo. EVs grant Downloaders a great Attack boost instead of Special Attack.
Surf is your primary STAB move, hitting Pokemon more reliably that Hydro Pump. It's rarely gonna be not powered up, due to the prominence of Rain in the current Metagame. Secret Sword is the secondary STAB move, hitting Pokemon like Blissey on their weaker Defensive side. Expert Belt allows for the switching of moves, and Ferrothorn is knocked out by Surf and Secret Sword, usually. Icy Wind is the Ice move now that the Hidden Power slot is occupied, and it really helps if you are threatened by Dragon Dancing Pokemon like Haxorus and Salamence, and lowers their Speed in time to get damaged again the following turn. Finally, Hidden Power Bug nails Celebi, one of the best switchins into Keldeo, extremely hard. Here's a damage calculation:
252SpAtk Expert Belt Keldeo Resolution (Neutral) Hidden Power (Bug) vs 252HP/252SpDef Leftovers Celebi (+SpDef): 65% - 77% (264 - 312 HP). Guaranteed 2HKO.
That's a pretty hefty amount for a Celebi to take from any Special Attack. And since Celebi will usually have taken damage beforehand, Keldeo usually can clean uo Celebi easily. Stealth Rock and Spikes damage help even more against Celebi.
And I think I talked more about Celebi then Keldeo...