I'm extremely picky about Notes apps. I've tested so many Open source as well as closed source apps. I'll be interested in what others are using, but the features I want are:

• Cross platform (Android, Linux, and MacOS)
• Universal format - markdown is a bonus
• Good task handling with checklist support

So what I've settled with is Obsidian (not open source) due to its simplicity of reading and writing to a folder hierarchy of plain text files. But since it sucks at task and checklists, I've been using Quillpad. It only syncs with Nextcloud at the moment, but there is promise of plain text file and bring-your-own-sync-solution on the roadmap.

Notesnook is a nice app, but since it's all E2EE, there is no plain text without exporting your notes manually. Shame too because it handles tasks and checklists very nicely.

Honorable mention: Acreom it's not open source yet, but that is on the roadmap. It is local first and plain text files on desktop OSes...but not on Android, meaning of you want to sync between your desktop and mobile you have to use their cloud. And I don't want to do that.

Joplin gets mentioned constantly. But it adds weird metadata to every text file and changes the titles of the files to some garbled hexadecimal string, which makes it impossible to know what you're looking at at the file level. And the task management/checklists is awful. Android app is bad too. I'm sure I'll get hate for hating on the FOSS golden child, but that's ok. This is simply my opinion. Like I said I'm very picky.