I loved hacking Windows back in 2000-s! It was super-hackable and had PLENTY of very thorough documentation (Petzold's "Programming Windows"!).

You could even do a lot of kernel-level shenanigans with relative impunity thanks to its layered design. You could do some amazing stuff.

As an example, SWSoft released container ("lightweight virtualization") support for Windows in 2005, before containers were even a thing in the mainline Linux. They did that by adding a layer of redirection on top of the kernel objects without having access to Windows source code.