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.