Ecco Pro for Windows and its predecessor, Lotus Agenda for DOS, which inspired a multimillion dollar open-source software debacle that inspired the book, "Dreaming in Code".
Ecco is still being used 20 years later and has been binary patched to support Lua extensions. Written by a four person team in Seattle.
Yes, I've bought every alleged outliner since and nothing compares. What's amazing is that Ecco had 100% reliable cross-device syncing in 1990s, both PC-PC and PC-PalmPilot. So it has enough metadata in the database.
TreeSheets (http://strlen.com/treesheets/) is an open-source cross-platform codebase (wxWidgets) with fast graphics, since it was designed by a game developer. That's one possible starting point for cloning Ecco.
Ecco is still being used 20 years later and has been binary patched to support Lua extensions. Written by a four person team in Seattle.
doogiePIM has been resuscitated by its founder, hopefully it will carry a torch for some of the ideas in Ecco and Agenda, https://bitespire.com/details_doogiepim.php