What is a good sprite toolkit for unity?

I am working on a platformer, in which all my artwork is already finished and polished. Now, however, I was going to start the editor work. I have tried some unity sprite toolkits, but none seemed to be as decent as expected. Can anyone point me to the right path? Can someone refer to me a powerful sprite toolkit which is not so difficult to use?

Thanks in advance!

I know this one and this, but the last one is paid.