I think GNU AGPLv3+ is the strongest copyleft license.
quote:"The GNU Affero General Public License is a modified version of the ordinary GNU GPL version 3. It has one added requirement: if you run the program on a server and let other users communicate with it there, your server must also allow them to download the source code corresponding to the program that it's running. If what's running there is your modified version of the program, the server's users must get the source code as you modified it."
So, if you are developer who likes Free Software, you better use GNU AGPLv3+ instead of GNU GPLv3+
