Keldeo is a beast. There's no other way to describe it that gives it enough credit for what it does. For example, my old rainstall team went 57-1 on Smogon back in April. That very same team is singlehandedly 6-0'd by Calm Mind Keldeo carrying Hidden Power Electric/Grass. It's going to have, and is already having, a huge impact on the metagame.
In my opinion, the best set would be
Keldeo @ Lum Berry (rain/weatherless) / Leftovers (sand/hail)
Trait: Justified
Timid Nature (+Spd, -Atk)
EVs: 252 SAtk / 4 SDef / 252 Spd
- Calm Mind
- Surf / Hydro Pump (I prefer Surf)
- Secret Sword
- Hidden Power [Ice] / Hidden Power [Electric] / Hidden Power [Grass]
However, a Choice Specs / Scarf set is perfectly viable as well, and can catch many people off guard. Specs Hydro Pump 2HKOs Specially Defensive Jirachi, 2HKOs Standard Ferrothorn in Rain, and other stuff that I'm too lazy to mention.
Not to mention, there are hardly any counters to it. The only really solid ones I can list off the top of my head are Psyshock Latios / Latias / Starmie, they seem to be able to switch into practically anything (bar a Specs super-effective Hidden Power after Rocks damage) and KO in return, regardless of Keldeo being at +1 or not. Tornadus is alright, but since it usually functions in Rain, and Surf OHKOs in Rain, there's not really a good chance for it to switch in, so it has to let something die and then attempt to revenge. However, Specs Tornadus can even OHKO Keldeo when it's at +2, which is nice. Other things wall Keldeo depending on what Hidden Power it carries. If it doesn't have HP Ice, Dragonite can switch in and set up all over it or OHKO with a CB Outrage. Jellicent can status it and Recover stall if it doesn't carry HP Electric, Grass, or the rare yet viable HP Ghost. The same goes for Slowbro there, but to a lesser extent. Basically, the only drawback to Keldeo is that whatever Hidden Power you choose, there's going to be something that walls you. The way to get around that is to pick the brand of Hidden Power that leaves you open to counters that the rest of your team deals with very well. Basically, give Keldeo due support, and it'll return your investments.
In conclusion, I see Keldeo as a top-tier OU threat that will hold a large niche in the tier, definitely be suspected, but ultimately remain one of the greatest threats in OU.