Not exactly a lecture series, but Andrei Alexandrescu has had some fantastic keynote presentations at CppCon. I don't even program in C++ any more, but I've watched these more than once, they're entertaining, approachable, and inspiring:

Speed Is Found In The Minds of People https://www.youtube.com/watch?v=FJJTYQYB1JQ

std::allocator Is to Allocation what std::vector Is to Vexation https://www.youtube.com/watch?v=LIb3L4vKZ7U

An actual lecture series I've enjoyed watching is "The WE-Heraeus International Winter School on Gravity and Light", which covers General Relativity using modern mathematical approaches: https://www.youtube.com/watch?v=7G4SqIboeig&list=PLFeEvEPtX_...

Again, I haven't done physics in 20 years, but I've found it surprisingly easy to follow because it is so clearly presented.