Two equally bad fallacies
Take a look at this photo by Ansel Adams: Notice that there's a mountain on the right side, but not on the left. There is no law of nature that everything is balanced. The conviction that everything is balanced has got to be one of the most exasperating logical fallacies out there. Liberals in America are not as outrageous as conservatives. Atheism is not as dogmatic as religion. Believing scientific consensus is not the same as believing the Vatican. The oppression women face in Saudi Arabia is worse than the oppression women face in Britain or America. If you aren't capable of looking at your own times objectively, consider America 200 years ago, and try to insist that the moral way to go lay somewhere between the slave-owners and the abolitionists.
What I find sets this fallacy apart from others is the self-satisfaction it gives people. The notion that the truth is in the middle makes people feel balanced and sane. How many times have you been in a legitimate argument with, say, a climate change denier, while some smug douchbag sits on the side, waiting to chime in with, "Scientists are self-interested too; there's no difference between the two sides," ending the discussion.
There's no reason to assume that people group their opinions around the truth, or around the best solution for society. Or to think that if one phenomenon exists, its opposite must also exist. Find me some evidence that two sides of a given debate are balanced, or drop that nonsense.
What fallacy annoys you the most?