Keldeo really is a great card, and that used to annoy me terribly because I didn't like to think about how much my Darkrai variant was going to lose to a ginger unicorn with rainbow highlights. Since then, it's dominated the meta, and my opinion of it is actually softer.
Though most known with it's partner in crime, Blastoise, which gives this card an insane damage capacity in exchange for consistency (I've seen enough :water: dropped on a Keldeo T2 to OHKO my Darkrai, and this happened two rounds in a row), Keldeo has many merits as a standalone card. I would say it's the only thing standing in front of a Landorus dominated format, as even without running :water: it can strike 100 against dat Weakness, and it's Ability is essentially a free, consistent Switch. You can kiss status conditions goodbye, move your attacker to the bench and charge him up with Dark Patches or Dynamotor w/out burning your 1 retreat, and eliminate Catcher stalls. Not too shabby, really.
But, is it really the best card in the format? I think not. I've consistently beaten Keldeo/Blastoise at every competitive event I've played at, which makes me think it's really just a balanced, good card. Great Ability, made imperfect by the :colorless::colorless: Retreat cost, Great Attack, but the 50 Base Damage is appreciated, and Great HP, although 170 is IMO a great number that allows for an array of creative, easy knockouts in my D-Krai/Hydreigon deck.
8.5/10 as a great tech card made awesome by the Weakness of better cards, lol.