I've been using PiskelApp for nearly 4 months so far. It's free, it's easy to use once you get the hang of it, you can download the offline version so you can use it without internet. Plus my pixel art is practically secured because my PiskelApp account is connected to my Google account. It may not be 'high quality' as much as other programs, but I'm still stickin' with it.
Perhaps for professional use:
Other pixel artists would also recommend Aseprite. I've heard about it but never used it. To tell you one thing, it comes with a free trial, but once that's over, you'll have to buy it to continue using it. I would suggest looking at this too, read some reviews written by the users. This may help for you to determine whether or not you want to have the program.