The Accuweather app, for free, gives you awesome features that you may have to pay for in several other apps. The interface isn't as sexy as some of the other ones, but it has full forecasts, multiple cities, and even radar maps with animations, and it all works very well!
For just radar I use StormTrak. It is awesome. For general weather I use MyCast and eWeather HD. I am a weather freak and live in the midwest. With a lot of tornados in the spring and summer, I can track storms easily. Currently I have 5 weather apps on my iPhone.
I spent several hours today downloading some weather apps most were free but I did buy a couple. Depending on what you are looking for these may or may not interest you.
My favorite is the Weather+ for which I paid $.99. It has the most features that can be customized. There are widgets that you can select from that provides temp, date, current weather and various forecast formats. The background is also one of the best I've seen. It has pictures that have motion such as water on a waterfall. I know this doesn't interest everyone but I have unlimited data and I'm home with broadband 75% of the time so it works for me. It is 4* with almost 15,000 ratings.
Another was the Fahrenheit Weather which is $.99 and has the temp on the app icon so you can easily see what the temperature is without opening the app. It's clean uncluttered and gives you an 8-day forecast. It has a 4+ rating with 1600 ratings.
On my wish list is the NOAA Hi-Def Radar for $1.99. If you want to see real-time high resolution map this could be what you are looking for and it also gives the current conditions and forecast. It is almost a perfect 5*.
Another on my wish list is the NOAA Radio for $.99 which is an app that provides the actual weather radio alerts. Once purchased any updating is free for life. A pretty good bargain for 99 cents.