So ... Which is the spriting software gonna be?

Make sure it is specific for sprites, allowing you to fill with gradients by using distance differences between pixels (there was an option for it in Deluxe Paint), showing you the previous frame in translucent upon your current one (onion skinning), and do whatever the software that Epic Games used did. No exceptions. It must be a flawless work.

First show me your animation, so I can tell whether it is coming out well. Then, if it is, continue to the next frame. We can do it!