I highly recommend Aseprite, which i believe to eb the ebst pixelart software available, but there are other options if you want them to be free or you want them to be general-purpose and not only pixel art.

Piskel

Online - Free - Pixelart-Specific

https://www.piskelapp.com/

GIMP

Download - Free - General Purpose

https://www.gimp.org/

Aseprite

Download - Paid (16.79€/19.99$) - Pixelart-Specific

https://www.aseprite.org/

Clip Studio Paint

Download - Paid (49.99$) - General Purpose

https://www.clipstudio.net/