I voted Gravity Nymph, Pandemonium, and Shard of Sacrifice for this poll.
Gravity Nymph - I realize this card is not overly effective against mono/dual decks (even if it could stall them), it destroys rainbow decks, almost regardless of when it's played. Unless you're carrying Sanctuaries around, the sheer amount of quantum control this card has even outdoes Supernova (three lost against two gained). Sure it dies easily, but give it even 1-2 turns and your quantum goes bye-bye.
I always thought the Unstable Gas Nymph was the worst until I met this one. >_>
Shard of Sacrifice - I fiddled around with this in the Trainer for a little bit (after a friend of mine told me about it), and it may be worse than dim/phase shields. Sure you need a set amount of life to play it, but with some control, it's almost impossible to run into a situation where you couldn't play at least one of them (luck, of course, being an obvious factor). I say it may be worse than dim/phase shields is because, last I checked, you could NOT remove the effect of SoS (unlike dim/phase shields, which you CAN remove); coupled with the fact that it HEALS you, you have both a stall and a healing card that is ridiculously cheap to play. The quantum loss I have yet to find to be an issue; and if you're running a mono-Death deck, it ISN'T an issue at all.
Pandemonium - To be fair, this card's ONLY OP if it's upgraded. But once it is, you have a relatively cheap card that hits all creatures your opponent has. But that's not the OP part; it's OP because of it can inflict just about ANY effect in the game. The only thing that holds it back is the luck factor... well, and the part where it might not do anything at all to a creature, but that doesn't happen often.
I'm surprised none of the polls have listed Parallel/Twin Universe as an OP card; considering it can easily be an Aether-equivalent to Chimera (sudden rush card to instantly defeat an opponent). Although I'll hold back on declaring it OP until I actually use it once again.