AoPS/Beast Academy

Art of Problem Solving is a company that has generated what I consider to be the most advanced and best materials for mathematically motivated kids.  Before they came along I was generating my own materials from math olympiads, Exeter and other places.  Now I enthusiastically use theirs.  My son is using Beast Academy in his homeschool and I have helped many kids with it.  As a remote class it is exceptionally high level and challenging and it is very helpful to have a person to discuss problems and ideas with.  The higher level topics are excellent.  The number theory book is on the lower level but that means that many grade and middle schoolers can access it.  Since encryption and quantum computing came around, number theory is now a practical class!