Gilbert Strang's lectures on Linear Algebra: https://ocw.mit.edu/courses/18-06-linear-algebra-spring-2010... LinAlg was the only maths course I needed in my interdisciplinary study program. I had struggled to grasp maths in high school, but these lectures really made it click for me and I passed my university's class with a B+.
When I was an undergrad math major, linear algebra was one of those general foundations math classes that students from all the science majors were in. Because of that they had to teach a lot of sections of it, and that meant grad students lecturing it. The class was kind of a mess, but luckily professor Strang had these lectures up online even back then (2003 probably). Who knows how I would have done in that class without them, they really saved me at the time.
I should revisit them, I remember them being great!