I'm going to join the chorus of Weather Underground, and I also use Storm by WU in conjunction with WU. Storm provides a nice interface for watching radar, tracking storm cells, and following watches/warnings. And you can jump straight into a location-based radar without a lot of menus or scrolling. It also can do notifications for lightning and rain near you. If you live somewhere that strong storms are a fact of life, Storm is an excellent secondary weather app.
I was once scared to death of strong storms and drilling into radar and how storms move and form helps me get over it.