For 2d bitmap sprites, I like to use an old piece of software named "Deluxe Paint" by Electronic Arts (ECA). But, it is no longer published and difficult (rare) to locate for purchasing, and sadly it has limits on the number of simultaneous colors. Ironic that despite its age, it still outperforms in aspects much of the newer bitmap editor software published today.
But I also like to use Adobe AfterEffects as well as Corel PhotoPaint. I especially like the "puppet warp" feature in Adobe AfterEffects which allows for advanced tweening. This can save an incredible amount of time instead of manually drawing each and every frame of a bitmap animation. There are probably several videos online (YouTube) which describe how to set up / configure Adobe PhotoShop (not the same software as AfterEffects), Corel PhotoPaint, Gimp and others, specifically to create bitmap pixelart / sprites instead of editing high-resolution bitmap photographs.
But what I have found is, almost any bitmap editor software will suffice. It takes time and practice to learn to use the software, but ongoing practice is the key. After practicing to an adequate amount on a bitmap editor of choice, then that same bitmap editor usually becomes the user's preference only because of established familairity.
So I would recommend "if the shoe fits, wear it" so to speak: choose a bitmap editor which fits (even if not perfectly fits, but closely or reasonably fits) with your current budget and current skill level and then continue to use it for the foreseeable future. Then in the future if either your bitmap art skill set increases or your budget for editor software increases, you can always at that time switch to adopt a different bitmap editor with more features. It is largely a matter of personal preference and personal taste, even as can be seen in the number of specific editors referred to in this thread posted by different users.
I hope this recommendation is helpful. Regarding recommending a specific editor, I would recommend Gimp even though I personally don't use it at this time. It's free, does not require an online connection (for either DRM or for an ongoing paid subscription), and as an added bonus it runs on multiple OSes (Windows, Linux) which does not lock you into a specific vendor's system (OS). And being community-driven in its development, it is unlikely that it will become "abandonware" or unsupported any time soon.
