If you could push a button to eliminate one cognitive bias, which would you choose?

Availability heuristic! ... It was the first one that came to mind.

Scope insensitivity so we would care more about existential risks.

This could have a chance of not ending well. Many of the biases are useful, even if they might hinder us under specific circumstances. It is very useful if society has a number of rational people, but if everyone was perfectly rational, I don't think society would work anymore.

A major benefit of biases, especially before advanced technology made our lives safer, was that in a dangerous, life-threatening situation you didn't have the time to evaluate all the choices and make a rational decision. You had to decide in the split of a second if you trusted that odd looking guy over there, or if the sound you heard was that of a tiger or not. This decision was made mostly by a large dose of prejudice, but could save your life.

The confirmation bias.

Confirmation bias first. Hindsight bias a close second - I'd like to be better able to decide if a mistake was actually a mistake given the information I had available at the time.

Learned helplessness, if that counts as a bias. Anchoring otherwise. Both are hard to overcome even one if aware of them, and decrease quality of judgement badly.

For the whole human race I'd dispense with availability heuristic, which would heal a political system and decision-making in democratic countries a lot (not due to individual politicians having radically better judgement individually, but due to decreased pressure from voters swayed by rare, but spectacular events).

Difficult to choose just one. The fundamental attribution error, probably. People would get along so much better if they didn't fall prey to that.

Not sure if this is a bias or not, but "magical thinking" -- the mode of thinking that implicitly ascribes a hidden 'essence' to events or objects. Different manifestations of this are: karma, homeopathy, "handmade is better than factory made", and so on.

Typical Mind Fallacy. Allows people to actually cooperate for once. One of the things I've been thinking about is how one person's fundamental mind structure is interpreted by another as an obvious status grab. I want humans to better approximate Aumann's Agreement Theorem. Solve the coordination problem, solve everything.

Negativity bias (from wikipedia: "Psychological phenomenon by which humans have a greater recall of unpleasant memories compared with positive memories"), or some other modification reducing negativity in my perception of the world for myself.

Maybe toning down scope insensitivity for the whole human race? (I'm not sure if completely eliminating it is a good idea - I don't know if having a brain which feels the correct emotions about arbitrary amounts of suffering would be too traumatic and lead to paralysis. Whatever level of modification that leads to correct actions)

The one that makes people think cognitive biases are distinct entities.

Confirmation bias

If I had to choose on an individual level, probably hindsight or confirmation bias would be the top ones.

If I had to choose for all of humanity, I would probably say any essentialist bias that makes people view essences everywhere. Negativity/Controversy seeking bias is another one i'd like to remove.

If it seems like we're stuck with a bad situation, reframe it as a positive to cope.

This one will probably kill me because it has prevented us from putting serious resources towards curing aging. 

Also (much less importantly) it means we do not put any effort into reducing the amount of paid work people are required to do, even as economic efficiency skyrockets. Shouldn't we be able to have 20-hour workweeks by now?

My real answer* to the question in the OP:

For myself, a bias that's probably related to narcissism -- a tendency to believe myself more impressive or effective or persuasive than I am reasonably likely to be, or to believe with unwarranted confidence that I would succeed where others have failed (in things I haven't yet endeavoured to master).

For others, the bias in which the truth value ascribed to a claim is dependent on the emotions it generates within us. Depending on whether the person is a biased cynic or a biased idealist, ascribed truth and positive affect could be directly or inversely correlated.

* My previous answer was partly true, partly a joke for its own sake, partly a comment on how the top biases people will come up with will be the most known ones, independently of the extent to which they harm our rationality.

Counterpoint: what cognitive bias would you most like to increase in magnitude?

I would go for the just world fallacy, as I think it is a broadly accurate and useful heuristic that is unfairly maligned.

Conjunction fallacy, especially the part adding detail makes a scenario more plausible but less probable

The reason is I enjoy screwing with people I love and respect. In this case, sci-fi writers. All your in-depth predictions of how life will be in 300 years are now invalid. Next move?