Balance is the worst card ever printed. I suppose it might only be the 6th worst cube card ever printed, but I doubt it.
I'd like to put in an honorable mention to the other sort of oppressive card: the blatantly over efficient. These cards don't completely stop the opponent from playing the game, but they give advantages to one player so unconditionally and/or at low cost, that it doesn't matter, you just win anyway.
This is where your moxes (other people pay 2 mana to ramp up, but not you! For you, that privilege is free), sol rings (see above, but better), ancestral recall (other people pay multiple mana to get a one card advantage, you get two cards at instant speed for being willing to spend a single blue mana), timewalk (when you spend two mana to ramp up, you get your mana back! And a combat phase! And another planeswalker activation! And. . .), mana drain (2 op blue mana shouldn't just counter a spell. Don't you know blue mana is better then every other color?), Mind Twist and balance (the flexibility of mindtwist, wraith of god and Armageddon in one card at half the cost!) all come in. I also put your manavault/black lotus/grim monoliths here.