<p>The school that will prepare you best for graduate school is the school where you are passionate about what you work on and where you find an environment that helps you stretch intellectually and emotionally. All three schools you mention can do that. I suspect, though that Chicago and Brown have an edge in areas like neuroscience over Pomona because they have the research resources that even well-endowed small colleges can lack in specialized science fields. But I also know excellent people in neuroscience who teach at small liberal arts colleges. And who knows whether your interests will change directions while you are in college? </p>
<p>You should go visit all three schools, talk to professors, and talk even more with other students in the areas you want to study. And then see which place(s) seem to stoke your passion the most.</p>