I keep personal notes in Dropbox. I formerly used plain text files, then I used my own syntax highlight format in GEdit. Then I started using Markdown in Sublime Text (with the "Sunburst" colour theme for making everything stand-out).

I tried vimwiki with the files stored in Dropbox but it didn't go anywhere. I liked the simplicity of just bunging stuff in a semi-structured .md file.

I worked with Markdown files in Dropbox for several years. However in the last year or two, I sometimes have occaison to read/write notes from my phone or tablet and Sublime Text is not available for those systems. I tried various Android editors like Markdawn and DroidEdit but nothing I was satisfied with.

These days I am slowly gravitating to Google Keep. It's simple but OK for my needs and I can use it cross-device - on desktop these is even a Chrome app which works like a desktop app.