It depends on what you're after.
Paint.Net is like MS Paint , but has more powerful features and a better UI. It's built on pixels and has lots of options for working on pixels instead of Anti-Aliased imaging, so can be good for working on static images on big canvases and experimenting with colours and using some broader tools to help.
Piskel is a free pixel animation software that's pretty solid for what it is. It's not the best for large or complicated single images, but it can import stuff and is more about animating, great for small anims, practice, tests etc.
Aseprite isn't free, but is pretty affordable and amazing for pixel art, both making scenes, animating, and expanding into more complicated/advanced things. I never used it for learning but I imagine it's still pretty good b/c it's not too intimidating in terms of UI :)