such a broad question indeed. i'd preface my answer by asking them if there is anything that they would like to know in particular, then mention the things that i could talk about, but ultimately tell them a few thigns about yourself (relating to dentistry or not) that 1) you are proud of 2) speak well to you character 3) are interesting to a broad range of people and 4) help to define who you are. You can even get creative and weave these personal attributes into how they brought you to pursue dschool. also...tell a story about yourself, stories always sell, take up time, are interesting, are easy to do if you practice.
going into a rant on your biochem professor that sunk your GPA with a D final grade is not what they are looking for by posing this question. In fact if i were the interviewer asking this question and got an answer explaining any shortcummings, i'd immediate show that interviewee the door. but thats just me.