A tablet. I find pixel art to be a whole lot easier with a tablet. :)
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I use an XP-Pen Artist 12, but I have some annoying driver issues with it. Before that I used a Huion New 1080Plus, which I quite liked. You don't need anything fancy for pixel art. The fact that it's a tablet is the main thing, so don't stress the options too much. :) The fancier things are mostly used by high resolution art like paintings and 3D modeling.