Plaintext works best for me.

One place is important like you said. But I've found it is helpful to have a distinction between high-level plan, like a feature roadmap for a product, and a focused plan for specific tasks you are working on NOW.

For personal projects, I have a TODO section in the README that looks something like this:

  [ ] Add support for...
  [ ] Do this next thing...

This tracks high-level tasks.

When I pick something to work on, it gets its own list:

  [ ] Figure out X API
  [ ] Add error handling
  [ ] etc.

In a team context, I've always used JIRA for the high-level tasks and when I start on a high-level task I break that down for myself into a plaintext list like above. I start my morning by revising that "focused" list.