One with lots of options, to make the images themselves. I used to use "Game Maker Studio", only for making the sprites, but that's now out of commission. I have to find something else.
Why don't you use the build in image editor?
And what means out of commission regarding GM:S?
Well any program will do if you have to right experience regarding pixel art. Could be more specific what tools are you looking for in the pixel sprite creating program? Whats your budget or are you looking for freeware/open source?
GIMP is always mentioned. Free, has the layers feature, and all the tools you'd need.
Piskel.com
Graphicsgale and Pro Motion.
paint.net or good ole MS Paint
My favorite is Pyxel Edit
http://pyxeledit.com/
Develop games in your browser. Powerful, performant & highly capable.
Those look like great programs. Thanks.
big up for Pyxel Edit aswell
I've heard good things about https://krita.org as well, but it might be catering more to digital painting than pixel art. I've never used it so it's here just for completing the list. <img src="{SMILIES_PATH}/icon_razz.gif" alt=":P" title="Razz">