Aseprite is real easy to use, and inexpensive. I've been using it for a few years, and new features always seem to be on the way.
$15 for full version, and a trial version is available.
https://www.aseprite.org/
Pro Motion NG is more expensive, but worth considering too as it is
packed with features. For example:
- Full tile-editing features (including a tile-map editor, so you can test your tiles as you create them)
- Pattern-drawing tool, for more complex tile patterns
- "Color Constraints" tool (helps ensure that your pixel art adheres to hardware constraints)
The full version is available on Steam for $40 USD, and can be bought directly through their website as well.
http://store.steampowered.com/app/671190/Pro_Motion_NG/
https://www.cosmigo.com
The free version is also available on their website. A comparison between free and full version can be found here.
https://www.cosmigo.com/pixel_animation_software/downloads