It really depends on what you want: I really like obsidian which is cross-platform and uses basically vanilla markdown which makes it easy to switch should this project go down in flames (there are also plugins that add additional syntax which may not be portable, but that's as expected).

There's also logseq which has much more bespoke syntax (major extensions to markdown), but is also OSS meaning there's no real danger of it suddenly vanishing from one day to the next.
Specifically Logseq is much heavier than obsidian both in the app itself and the features it adds to markdown, while obsidian is much more "markdown++" with a significant part of the "++" coming from plugins.

In my experience logseq is really nice for short-term note taking (e.g. lists, reminders, etc) and obsidian is much nicer for long-term notes.

Some people also like notion, but i never got into that: it requires much more structure ahead of time and is very locked down (it also obviously isn't self-hosted). I can see notion being really nice for people that want less general note-taking and more custom "forms" to fill out (e.g. traveling checklists, production planning, etc..).

Personally, I would always go with obsidian, just for the piece of mind that the markdown plays well with other markdown editors which is important for me if I want a long-running knowledge base.
Unfortunately I cannot tell you anything with regards to collaboration since I do not use that feature in any note-taking system

Adding another +1 for Obsidian. I've tried a whole lot of note-taking apps over the years and Obsidian is the best I've ever used by a pretty significant margin.

That said, there's one caveat to note about self-hosting: if you're on iOS, there's not much of an option for syncing besides their paid Obsidian Sync service. I think there are some hacky workarounds for this, but they don't seem great.

Another option to consider is Joplin. I used it before Obsidian, and while it's not nearly as slick as Obsidian, it is fully open source, cross-platform, Markdown-based, and supports syncing with a variety of protocols out of the box. I had it set up to sync to a directory on my NAS using WebDAV and it worked reasonably well across all my devices. It also feels more Evernote-like to me than Obsidian and there's a built-in option to import an ENEX file if you want to move your notes over.