What are the best US colleges if I don’t know what I want to study yet?

The idea of the "best" college is a little difficult to determine. Websites like U.S. News and World Report provide annual rankings of universities and colleges in the United States (linked below). However, that site's ranking methodology has been critiqued as not fully considering some factors students may care about and over-weighing others to the point that colleges and universities can try to spend money in certain areas of the school to "game the system" and increase their rankings.
To determine the best colleges and universities for you, it is important to determine what you care most about in an educational experience and as a person. Do you want small classes? If so, you should look at institutions' faculty-to-student ratios. The size of the college may also be important for you to consider. Some people like the closeness that is often felt by students of smaller schools, while others believe more opportunities are available when there are more students on campus.
Since you don't know what you want to major in right now, I would encourage you to consider schools with a large number of majors available. This will give you a lot of options and a wider variety of courses to explore before you make your decision. If there's a certain area that is of interest to you, you could focus on schools with well-regarded departments in that field. For example, if you know you want to study something environmentally-related but are unsure if you want to study environmental studies, marine biology, or geology (that's not an exhaustive list; some schools have other environmentally-focused majors), you could look more specifically at schools that focus a lot on the environment.