Best resources to supplement an underwhelming Gen Chem, Bio, Physics, etc. curriculum?

Hi guys!

I'm an undergrad at a state school and honestly, I have a concern regarding my classes. I'm working on prereqs for the MCAT and for my degree, and although I probably won't study for the MCAT anytime relatively soon (freshman), I do want to make sure my understanding in prerequisite classes is strong and will build me a good base for the future. One of my concerns is that I'm not getting the rigor that I want from my current classes. I know a couple of schools that have hella rigorous prereq classes which are rough, but probably dig deep (in a good way). I wanted to ask anyone if there are any recommendations for supplementary resources in classes like gen chem, bio, physics, stats, etc. that you guys found helpful? If it's rigorous and in-depth the better-- I really want to get a deep understanding in these topics. Textbooks, MOOCs, or other online resources, I'm open to them all. Thanks so much in advance!