I greatly recommend Krita, it has amazing pixel tools and is under active development. Krita is free and open-source. It can be a slightly bit difficult than average to learn, I would compare it to Photoshop in this regard, it is near industry-standard level software, but it's worth the trouble.
It has pixel brush engine, pixel orthographic perspective guides and animation support and some other nifty tools, just google "krita pixel art guide" or something along those lines. make sure you look at guides from 2017-18, because Krita received a lot of large updates lately so any older videos will be outdated.