I really enjoyed Tim Roughgarden's 'Algorithms' specialisation. The first half are actually on Stanford's online learning platform for free.
That was then hosted on Stanford's own Lagunita platform (based on the edX platform I think.)
Stanford then split those 2 courses into 4 on Coursera's new platform - the same material I believe but packaged differently.
Stanford then closed its Lagunita platform and those original 2 courses are now offered on edX.
A similar story with Alex Aiken's excellent Compiler course and Jeffrey Ullman's Automata course too. I'm not even sure if they are still available on Coursera or not.
[1] https://www.edx.org/course/algorithms-design-and-analysis
[2] https://www.edx.org/course/algorithms-design-and-analysis-pa...
This is always a difficult question, because it's always "high school level math" which is rarely true. Then again, some algo courses focus highly on math and proofs while others focus more on implementation.