"What is a choice you regret making to this day and why?"

There is only one reason that someone should live with regrets; that is if he or she did not learn from an experience. Hindsight is always 20/20. If we are mindful, every experience in our lives, whether it be positive or negative, is just that--experience. All we can do is make the best decision possible with the information we have at hand.
It's too easy to simply regret--"Oh, I should have chosen differently." That's futile. You did not know what you now know at the time the decision was made.
In reality, one should learn from mistakes, and simply take them as information--"This choice did not work out how I intended it to, but I can take the information I've gathered from my actions' consequences and apply them to my future."

Without mindfulness, or without the desire to learn, grow, and better oneself, there is regret... But only in those circumstances. Fill those voids and you eliminate regret.