I love outliners. Outside "work’ apps, (Powerpoint/ Word) they are probably the application I use the most.
I have used Omnioutliner from the time it came free with a G4 12inch powerbook. It is very flexible, but is also becoming too feature rich (and expensive) for my needs. I also use Zavala (as @liminal has linked to) and Cloud outliner Xwavesoft - Cloud Outliner 2. Simple and effective.
Always on the lookout for more 
I don’t tend to use mindmap software for outlining , but I will output an OPML file into a mindmap.
There are some good threads here
Current state of Outliners - Software - MPU Talk
Zavala, shiny new outliner app to track. Available on macOS iPadOS iOS - Software - MPU Talk
New App: Bike by Hog Bay - MPU Talk
EDIT:
Apologies to @karlnyhus I see he had already linked to the last thread