I hesitated to touch that one because I don't vote and it's not a popular stance. The "none of the above" option mentioned by carbinemike does make some sense and it's the only one that I'd pick. I've been convinced for a while now that voting is only a puppet show, a sham put in place to pacify and entertain us. It's not a tool to empower the people, but to enslave them better by distracting them from what's really happening. Genius, really: make them believe that they have a voice. Decent candidates would never make it to the top where visible and less visible interests (all nefarious) own the presidency, the media, the legislature, etc. Nothing ever changes - it only takes on different forms and names throughout the ages.
So to me NOT voting is telling the system that I'm not buying it and refusing to endorse another professional liar. The lesser of two evils won't do. And the reason why dictators force people to vote is not because they demand that a civic duty be performed, it's to show that disobedience or disbelief in the system won't be tolerated. When they don't like the results of an election they just cancel it or launch a coup.
ETA: I don't believe that there should be a bar to pass to exercise a right either. I'm a lot less educated on politics than Obama yet I truly feel that he's wrong and I'm right, so that my vote is no less worthy than his, for example. I just had the same discussion with a coworker about another right, the one to bear arms, and I refuse to accept mandatory exams and training (even if I'd have no trouble). So I don't think we should test voters on their knowledge (again, many very misguided people would pass) - I think that our society should change. We should raise people with a strong understanding of what the Constitution means and a will to defend it. We should raise them to be decent people with integrity and a sense of purpose. None of this is done today (it goes again the Grand Agenda that's been in motion for decades) and that, in my view, is what needs to be fixed.