I realize this question is contrived, but I figure it might provoke some fun discussion, so here goes:
If you could push a button and have your brain modified to precisely remove a cognitive bias (and have no other unnecessary effects—most convenient possible world), which would you choose? Why?
What if you were choosing for the whole human race?
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).