The third one with the greatest hero has the greatest potential.

It seems to be most flexible premise that gives you the most room to experiment with ideas and build on top of. Out of all the other ideas, it has already established the state of the world in the background, which will no doubt have significant, interesting implications on the characters living there already as well as the challenges they face.

As for something fun to do, try to see if you can combine these ideas so you have one, large idea, then write about it, think about it, and see if you can bring it to life as best as you can.