I've got a few, but probably my favorite is this one I started in middle school and have been cleaning up and editing since. It's a sci-fi, and I at least think I follow actual science pretty well, but I came up with the general idea in middle school, so please bear with me.
My story starts in 2021 when a two shooting stars collide in the middle of a meteor shower. One of those meteors was actually an alien weapon containing DNA from the stars, and the collision spread the genetic information around the western hemisphere where it combined with the DNA of humans. Basically now, super powers are popping up on the Americas.
As governments struggle to come up with a solution, the supers split into two main groups: one that believe the powers should be used to help the powerless, and those who believe society should change with the time and that those with the "gifts" as they call them should be on top.
My main character is a high school girl named Nova. Her power is that she can literally tear apart atoms and molecules and reattach them however she desires. The only problem is that, because her powers work on such a small level, it takes a while to change large things. It isn't very helpful in a fight, but in every other aspect, makes her pretty much a god. And when combined with the fact that she is a leading member of both groups and incredible at keeping secrets, the fate of the world can be whatever she pleases. That is, if she can keep her secrets and come up with a plan before her friends on opposite sides start killing each other and targeting her.
My main problem was that my first draft had about 7 different story lines that all came together at the end. It was hard for even me to keep track of everything. I'm currently editing it down and hopefully will get it published in the next few years.