Notepad++ was my go-to Windows text editor. It can open multiple files in tabs, has syntax highlighting, remembers and loads last session, and has lots of plug-in support.
Firstly, it's useful to install Plugin Manager. It's in the form of a .DLL that goes into the standard plugins directory, and an .EXE that goes into the updater directory.
MarkdownViewerPlusPlus provides a preview pane that renders your Markdown file (though not so prettily. Markdown_npp provides syntax highlighting.
You can select by columns! Either with Alt + Ctrl + arrow keys or Alt + leftmouse + drag.