Hard question to answer, but if I had to pick I’d want to say real analysis (kind of like a very in depth calculus). Calculus as a whole is probably the most important invention/discovery in all of science, and its foundations are just so beautiful. It was the hardest class I’ve ever taken (class average was a D+ 🥲) but also the most rewarding, because it’s really helped me with mathematical intuition and just a sincere appreciation as to how elegant math can be.

That being said the most fun class for me recently has been group theory. It’s very satisfying and symmetrical groups are so cool ☺️