I'm using ... Notepad++. Yes, I'm a dinosaur :D

I've tried many others, VS, VSCode, Atom, Emacs, Vim... and a dozen other free editors I don't remember already, but it feels like other editors don't do a bit more but by an order "heavier" or "clunkier". But most likely just because I couldn't set them up properly to make use of their features.

Apart from that Notepad++ does it's job well. Really helpful autocompletion. Smart spellchecker (detects strings and comments). Good syntax highlighting and code folding, many useful tools, such as "replace X to Y in all open files", "trim all lines" or "change line endings to Z". I'd still make a big use of "go to definition" and other tools that require analyzing the code (like CodeTools for Lazarus), but for now I just don't have the time to tinker with settings to properly load a Unity project with no promise of result :)