I use a horrific mix of:

• Markdown (most of my notes these days, supplanting my previous use of plaintext.)

• Single-file HTML (when my use case for fancy documents exceeds markdown's limits. While you can embed HTML into markdown, renderers vary in what they allow or sanitize, and vary in how you can (ab)use style tags. I've even taken to looking up CSS rules for printed documents, since my intended target is often pdfs or actual print at that point. Occasionally, JavaScript!)

• Google docs (Android phone nonsense!)

I'd mutter something about "using the right tool for the right job", but that's probably giving myself too much credit.