I'm using Trilium notes. it's simple enough and does what i need. Used to use Obsidian but wanted something open source, and with Trilium you can self-host the sync server for free (even comes with a handy web-ui).
Note that it is much simpler than obsidian, but for me it's plenty. It was easy to import my obsidian vault into it, and it allows exporting as .md files which work fine back in obsidian too.
Recently the dev said he's putting it into maintenance mode, so no new features will come to Trilium. There's a community around Trilium Next that wants to keep expanding it, but personally i hope Trilium stays as it is and is maintained for a long time.