Highly recommend Workflowy (https://workflowy.com/). Its core idea is just recursive lists. Everything is a list, and every bullet can be expanded to become a top-level list. I've found this to be the most natural way for me to organize information, and there is a satisfying symmetry to it.

The developers are also clearly careful when adding new features, since they always compose well with existing functionality and create a multiplier effect on productivity.