Whatever you like. Something that you like will probably be easiest for you. Sure we can list some fancy major that is hard, but if you don't like it you will do bad and not have as good of a chance for dental school.
With that said, biology is the most popular. There are some chemistry, biochemistry, engineering, english, psychology, etc majors too.
A biology major usually requires courses for the major that are also pre-reqs for dental school. So you are not taking extra classes. Like say if you major in english, you will be taking mostly english and won't have as much time to take upper level biology classes.
So if you like biology, do biology. If you like chemistry, try a chem or biochem major (although these are harder).
If you are unsure bio vs chem. Just start off you first semester as undeclared major. You can start taking basic classes like general chem and general bio and an elective. These classes are the same for most science majors. Then declare a major in 1 year once you decide. Lots of people are undeclared or change majors their 1-2 years.