Skies can be tricky. The best lighting for human subjects is a slight overcast or near dusk/dawn, which can leave your sky a little washed out or darkening. Or if you wait until sunset, you have to get creative with lighting your main subjects (unless the sky is your main subject). Occasionally, you luck out and can get a great sky without sacrificing whatever else is in the picture.
Here are my choices for One Word Photo Challenge: Perfect Sky