Over at The Sweet Setup, CARROT Weather was chosen as the best general purpose weather app for iOS. It has Dark Sky and Weather Underground support, a great design, and a fast, customizable watch app. While there are a lot of great apps, CARROT Weather is the one that stands out above the rest.
You can buy CARROT Weather on The App Store for $3.99. It also has an optional membership fee that starts at $2.49 per year.