Biochemistry. If you look at the stats of students applying with Biochemistry majors (it's very very low). Not only applying to dental school, but compared to a lot of majors in university. Secondly, it's a hard major, thus the reason why less people decide to pursue it. Thirdly, biochemistry courses + labs (you will be doing biochem labs if you're a major), are a bonus (IMO).
Although, I'm not sure if adcoms pay a lot of attention to the majors themselves as opposed to the courses you've taken.
Maybe this is just my one-sided opinion, maybe others with more experience can chime in.