Challenging universities worldwide?

I'm seriously trying to push myself academically and I'm thinking about applying to some of the most challenging universities out there. Doesn't matter if they're in the U.S or international. Any idea what universities are notoriously known to be difficult for study? Bonus if their curriculums are particularly mind-boggling for a science student.

Here are a few universities with rigorous science programs that might be aligned with what you're looking for:

1. Massachusetts Institute of Technology (MIT): MIT is widely acclaimed for its science and engineering programs. The MIT curriculum is renowned for being intense, even for students with a strong background in science and math.

2. California Institute of Technology (Caltech): Caltech is known for its small class sizes and an academic culture that emphasizes innovation and intensive research. Its science-focused programs are some of the most challenging in the world.

3. University of Cambridge, United Kingdom: Cambridge's Natural Sciences program allows students to explore different areas before specializing. The workload is heavy and the learning pace is fast.

4. Swiss Federal Institute of Technology (ETH Zurich), Switzerland: Renowned for its rigorous engineering and science curriculum, ETH Zurich also emphasizes research-based learning.

5. University of Tokyo, Japan: Widely considered the most challenging institution in Japan, the University of Tokyo’s science departments conduct cutting-edge research.

6. Tsinghua University, China: Another university that's well-regarded for its demanding science programs.

These universities have a rigorous selection process, and their academic programs are challenging. However, keep in mind that the challenge isn't just in the content of the courses, but also in the teaching style, method of examination, and the high expectations for student initiative and independence.

Also, remember that while applying to challenging universities can be a great way to push yourself, it's equally important to apply to schools where you're confident you can thrive and succeed. It's not just about getting in—it's about finding an environment where you'll be able to excel and achieve your goals.