A lot of people seem to be recommending OneNote. I've played with it a few times, but I don't mesh well with everything being nested boxes. I haven't seen anyone mention Workflowy here. In Workflowy, everything is nested outlines, which seems to match my brain better. Unfortunately, it doesn't support images or even much formatting yet, but it is practically frictionless to navigate without your hands leaving the keyboard. I use nested tags in Evernote to get a similar outline effect, but Evernote seems to view the feature as an afterthought. Only the desktop client supports nesting, and it doesn't integrate well. For instance, if I click New Note, I expect it to populate the tag field with the currently selected nested tags. Instead I have to manually pin every note into the tag tree. Tedious.
