FireAlpaca is open source and really user-friendly! it has a lot fewer controls than Krita, and the interface is set up a bit more minimalistically, so it's better to start with imho.

i mostly use Photoshop myself though. it sucks that it's so expensive and there's no way to pirate it anymore, but it really is the best for pixel art and graphic design, and it can mimic MS Paint really well.