I'll start. For me, I think the most impactful thing I've ever built was an internal application for a FX trading desk that eventually went on to handle billions in daily trades.<p>It didn't use any fancy frameworks, just plain old CRUD on Java.
Sometimes it takes a book or a course (or explanation from a mentor) for a topic to finally click for you that you were struggling with for a long time.<p>For me, it was Stanford's EE261 course that made Fourier Transform click for me. Here is the link: <a href="https://see.stanford.edu/course/ee261" rel="nofollow">https://see.stanford.edu/course/ee261</a><p>Similarly for deep learning it was fast.ai courses.<p>For programming it was How to Design Programs at www.htdp.org.<p>Your topic of choice may be anything, not necessarily CS.