Notion's UI is so genius. It's clean, intuitive, flexible. It's like everything I love about Google Docs, Quip, and Workflowy but with Slack-like slash commands, mentions, and integrations.
Free for now, but will start paying when I run out of blocks (the usage isn't as high as I expected, only at ~500 out of 1000 right now, though I have imported 4-5 years worth of notes).