I'm an international student and I'm thinking of applying to top US colleges. I searched everywhere but no one seems to know if I should take the IELTS or TOEFL (besides the SAT). Some of my teachers say US colleges favor TOEFL, others say there's no difference. If the second is true, I prefer taking IELTS. So which one should I take?

In general, both are acceptable. This article gives more information about how to find the English as a Foreign Language testing requirements for colleges, which you may find helpful. I did a quick check on some of the top colleges and universities and found the following:

Accepts TOEFL or IELTS (possibly other tests as well) - Amherst College - Middlebury College - Swarthmore College - Princeton University - University of Chicago - Yale University - Columbia University

Only accepts TOEFL (according to website) - Wellesley College

Doesn't require either - Williams College

You should definitely check with the schools you're planning on applying to before making firm plans one way or the other, but for the most part IELTS is as widely accepted as TOEFL. For more information on applying to colleges in the US as an international student, be sure to read this article.