The highest-scoring one I remember from last year had to do with how much environmental damage certain household products, like shaving cream, caused. The students tested this in a couple of different ways.

What gets the high score isn't really what the topic is, it's the originality that goes into coming up with an idea and finding ways to test it. Being careful to control the variables is important. Having good graphs to show results is important. Knowing what you're talking about is most important.