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.
Most people say that you should major in whatever you're interested in as it's not something adcoms really pay attention to, for me biochem covers a good amount of the prereq courses I need. Ultimately, it's probably up to what you want to learn about...