Adobe Photoshop is the go-to app for bitmap graphic design for games. It's powerful layer options and effects allow you to apply effects to many objects that would otherwise be too time consuming to be done by hand. It caters very well for pixel editing and has thoughtful shortcut keys to optimise your workflow.

It was my go-to app for app design but now I use #Figma or #AdobeXD because they allow easy export to multiple screen densities, they have very good vector design tools, they also support grids, margins and frames in a way that's optimised for app design, and they have integrated prototyping.

I did try #Gimp for a couple of years which had good performance but in the end I found the layers implementation was very clumsy and it lacked the thoughtful keyboard shortcuts Adobe Photoshop has.