If you desire something cross-platform, or want an integrated spell-checker, I can recommend CherryTree by Giuseppe Penone (AKA giuspen) and Evgenii Gurianov (AKA txe).
CherryTree is available for Windows, Linux, and macOS. There’s even a third-party Android app called SourCherry that allows you to read CherryTree files on Android and perform some editing.
CherryTree include import functionality to import data from a variety of formats.
CherryTree is feature-rich, but still fast. The authors completely rewrote CherryTree in recent years, greatly improving it.
CherryTree includes powerful search functionality. I do hope they improve the UI/UX for search in the future, as I find it cumbersome (and is the only part of CherryTree that can be a bit slow).
CherryTree is open source freeware. Donations are accepted here.
CherryTree screenshot:
image source: https://www.giuspen.net/cherrytree/
