Most common cognitive biases?
This question is not answerable without having a measure of what the most common day-to-day scenarios are. However, I can try a guess: since most people interact in a public, anonymous setting on a daily basis, I would think the fundamental attribution error is most probable. This is especially true while driving -- "did that idiot just cut me off?!"

However, my estimation of fundamental attribution error as the most common cognitive bias may itself be an instance of availability heuristic.