I use just plain text file opened in plain text editor with following conventions:

  # Topic
  [ ] A task. @tag #ticket
    [ ] A subtask.
      [ ] A sub-sub-task.
  [+] Completed task.
  [-] Failed task.
  [.] Partially completed task. WIP.
  [!] Urgent task.
  [^] High priority task.

Easy to store. Easy to share. Easy to edit at any device with plain text editor.

The only feature I miss is grouping of sub-tasks. For example, I may want to group all subtasks from all projects(topics), which I need to do at place X (e.g. at parents home), so I will prepare for them. I cannot select them all without using of SQL engine or grep like tool. If editor can highlight a search term, it helps, but for long lists it easy to miss.