My vote is for minimal options in a WYSIWIG, configurable in the admin.
Bold, Italics, Underlined, Link, Image (just URL), Quote, Code.
Think that's it. Code is pretty useful for coding forums (would be useful here for example) but doesn't need to be enabled by default.
I've not really seen anyone use the text colour options in forums I've run unless they're being silly, so no real need for those. Plus they make posts look ghastly and we can't be having with that on such a lovely piece of software 🙂
If the editor button output parsed as markdown behind the scenes then all the better as you can switch between having the editor buttons on or off either forum-wide or per-user if you wanted, and we're all about the plugins here at Flarum Towers so you don't want removal of the button-laden editor plugin to break everything 😉