Minimal theme, without a doubt. It increases customization of the editor immensely, I can hide whatever I don't need, focus mode is neat, and generally it feels not too far from vanilla obsidian experience
I loved Gruvbox from the moment I first tried it, but couldn't live without Things' tasks status, so... I'm using Things as active theme, but I've added Gruvbox's entire CSS as a snippet. Beautiful beast.