Whatever default explorer for the OS.
That and something like WizTree for a nice visual representation of used space, which helps in finding whatever is eating up your storage.

The Files app for Windows is pretty good. Last time I used it regularly, it wasn’t a simple process to make it the default. Maybe that’s changed…

File explorer works just fine or powershell via the new terminal app works perfect too.

For Linux systems I’ve flipped all over the place, so I don’t really have a preference. Aesthetics and themed explorers are nice but at the EOTD I only use them for dragging convenience and easy access to folders that I keep in some sort of organized manner.

If I need to do “power moves” it’s likely just going to be via the terminal again

SpaceFM. Customisable, has plugin support.

Dolphin