Visual Studio Code is the best I've found so far. OP says they're using Windows, so they'd want to follow the link, but for Linux, you can usually find it in your package manager (it's called code for pacman users).
Here are some editors I've played around with while looking for plain TeX support. All of them work on Windows, Mac, and Linux too! If you try one of these other GUI editors, make sure the correct filetype is selected.
- Vim: Pretty good. Good syntax highlighting with reasonable colors. And although I haven't tried it, vimtex looks like a good plugin that'll probably add more features for plain TeX like command completion and compilation. I definitely know that you can rig Vim to run your own compilation commands/sequences.
- Emacs + AUCTeX: Emacs is an alternative to Vim. Others have mentioned that AUCTeX makes for a good environment, but already knowing Vim, I find it hard to switch to another editor with a similar learning curve. If you're an Emacs fan, go for it; otherwise, I'd recommend giving VS Code a try first.
- TeXstudio: Syntax highlighting issues. Too hard-coded to support LaTeX only, but command completion does exist for plain TeX.
- Sublime Text: I think I might have had to install a plugin. There's syntax highlighting support that's decent, but no command completion. The default coloring for syntax highlighting is a bit ugly in my opinion too.
- Atom: Nothing out of the box, but you get some decent syntax highlighting after installing the plugin
language-tex. No command completion. It looks like there exist spell check plugins that are still in beta, but I couldn't get them to work at the time of writing this. LaTeXTools might be another plugin of interest (it's recommended by TUG).
About VS Code
VS Code (Visual Studio Code) has plain TeX support after installing the LaTeX Workshop extension (Left menu -> search for extension).

Make sure you have TeX selected as the language mode once you have a file open (bottom status bar).

Here you can see some of the syntax highlighting and command completion:

You also get spell check through the Spell Right extension (supporting multiple languages). Once installed, it takes a little configuration---read a little further down the Details page of the extension for your operating system. I had to run (you might also have to install hunspell):
ln -s /usr/share/hunspell/* ~/.config/Code\ -\ OSS/Dictionaries
and then enable English spell checking in the editor status bar.
I might come back and add more once I figure out compilation features in VS Code (if you've got it figured out, just edit my answer), but I really like what I see so far. I was dumping this as I was learning about it. I hope it helps someone else looking for good editors.