Obsidian

Your notes live as plain Markdown files on your own disk, not in someone else's cloud — which means Obsidian is arguably the only app here you could still open in twenty years regardless of what happens to the company. The graph view linking notes together is genuinely useful once it clicks, but the setup isn't trivial: new users regularly report it taking months to feel natural. Worth the investment if you're building a long-term personal knowledge base solo.