We've been incredibly happy with the choice of Markdown as the composition language for Zulip:

• Everyone can write it, since it's just like email and many other products use it.
• It has a WYSIWYG type experience, so we don't feel the need to integrate a fancy editor like Quill.
• There are mature markdown processors for every programming language. Having compatible, high-quality implementations for Python and JavaScript is important for doing local echo properly (see our linked Markdown documentation for details of why is necessary).
• Rendering to HTML is fast, easy to do automated tests for, and easy to debug.

We have had to customize our Markdown implementation somewhat for the chat context, because we don't have the guarantee that content is not split across multiple messages (a good example is that you don't want the "automatically change numbered lists to start at 1" feature in default markdown; you want to restrict that to numbered lists where all the numbers are 1, for example) .