A passport--it's the gold standard of ID. A U.S. driver's license (or state ID card issued by the same agency) probably won't be accepted anywhere other than in the U.S. and Canada. But a passport is accepted worldwide.

Also, for those of you concerned about privacy, a passport reveals less information about yourself. My U.S. passport simply gives my name, citizenship, date of birth, gender, and place of birth (state and county). It does not disclose where I live, my Social Security number, my driver's license number, my weight, my height, or my eye color.

While each U.S. passport does have a unique number, I am not aware of any commercial database that allows you to submit a passport number and obtain information about the person who holds that passport. That isn't the case with driver's license or Social Security numbers. At my previous job, I had access to computer databases that would allow me to input either one and get a bunch of information about the person to whom that number was assigned. (Forget about privacy laws--the work I did fell under an exception contained in every privacy protection law I've ever read. And no, I wasn't working for the government.) I tested these databases by looking myself up, and it was impressive just how much information they had on me.

Anyway, these days, unless there's a need to verify my ability to drive a car (e.g., if I'm renting a car), I usually present my passport when asked for ID.