I think the reason you're having difficulty determining a proper crop for this photograph is because there's nothing really for the eye to focus on. Sure, it's a pretty shot of some clouds. But anyway you crop it is going to look pretty much the same. Just some pretty clouds.
I would love to see something as a focal point of the shot. Something in the foreground. A bird. A plane. Superman? Just something else to gold my interest.
Crop this shot however you'd like, but it wouldn't really change much. The eye really needs something more to focus on. I don't want to sound mean, I just want to help. Hopefully I have been able to help.