Obsidian

I've used Obsidian for a couple years. It's a very good note-taking app. Vault (a paid add-on) makes syncing across devices effortless, but other cloud solutions also work. Composing in markdown's fast: your hands never leave the keyboard. It stores notes in markdown, so there's no lock-in: you can open them in a text editor. Once a note is synced to your device, you can reliably get to it even when you're offline. Quirks are in tables (a bit clunky) and the outline (hard to find on phones).

OMG! Love this so much! I'm a Tinderbox for Mac user; although this is not Tinderbox - never thought anything like this was possible on a phone. Bar none, this is the Best note-taking, brainstorm, research tool for Android. A slight change of perspective required to simulate Tinderbox's map view, but it is close enough. Plug-ins further enhance its versatility. It is very techy though, so be forewarned. Logseq, a competitor, is still in its infancy, and still very rough around the edges.

Really, really good app! The slow startup speed is fixed now! Both, the desktop and the mobile apps start pretty fast. The only thing to improve, IMO, is the finicky "Send to" Obsidian on Android. It often reloads the app every time it's used, even if I just switched from the app (its battery settings are set to "no restrictions", so it shouldn't be killed). Another killer useful feature on mobile would be a quick note feature!