Top 3 daily driver weather apps:
Weather Underground Downloads for multiple platforms
Free, Universal on iOS. WU subscribers have ads removed ($10/ur or connect your weather station and get it for free)
"If I had to have only one weather app", it would have to be this one. Part of that is more heavily weighted because I operate a weather station at my house that updates to WU every 3 seconds, and this app allows me to keep watch on it. But beyond that, I've found this app, while not the prettiest weather app, has the best overall utility. Excellent radar, excellent written forecasts. They've recently added nice graphs to show forecasted trends from the next few hours to ten days out. A big deal here is that they don't rely upon the National Weather Service forecasts. Those forecasts don't translate to regular people very well because they cover what can happen within a large reporting area. So "30% chance of rain" is true if it happens anywhere within your corridor. But that's not helpful if you're not near that part. WU forecasts build upon the NWS forecasts and tailor them to smaller areas, so they're more likely to be accurate for your particular spot.
Another benefit is that WU's network of reporting stations has like 100k more stations than the NWS, so there's more likely a station geographically near you. For example, Boston's NWS reporting station is at Logan. Logan is on an island in the bay, so it's more likely to be cooler and windier than the city. In the winter, the surrounding water can influence when the airport is more likely and less likely to have snow versus just a couple miles away. WU has stations all around Boston, which they use to suppliment NWS stations, which they also have access to (and which tend to be more accurate and professionally maintained—but that's only useful if you're very near them and are geographically similar!)
So, if I'm in the U.S., I want to make sure my weather app has access to WU data and forecasts.
Free, iOS with sepereate iPhone and iPad apps. Offers in-app purchases that remove ads and add features (one subscription works in both iOS apps). I bought 6-mo subscription to Storm Tracker, which is really well done. Android version also available.
Best radar app I've seen yet. Forecasting is mediocre in my experience. UI is subpar. Oh, but that radar! It's so nice. Their predictive reporting is pretty awesome, too. Slider allows you to drag through projects radar for the next hour (not unlike Dark Sky below, but more interactive). Has pro-level tools, including the ability to draw on top of the radar map and save and share the image.
Intellicast is a WSI subsidiary. WSI provides professional weather data to weather professionals (like the to the meteorologists you see on TV) They also make weather apps for local TV stations, like NECN's. The NECN app is terrible. Intellicast's own app does less, so you have to interact with the terrible UI less.
iOS Universal (portrait only)
$3.99
Use this if you want to know if you're going to be rained upon in the next 60 minutes. Beyond 60 minutes, they just pass through the NWS data. See above for how NWS data may or may not be immediately applicable to you. They appear to be working on creating better data (see Lines below).
Other weather apps on my phone:
http://forecast.io/lines free web app (ads at bottom, saves to iOS homescreen).
Really neat web app. Shows the all the major weather prediction data models that pros use to make predictions, and allow you to see each model's prediction for your location. Predictions for temp, precip, wind, humidity, pressure.
$1.99 on iOS, universal (only portrait mode, though)
Takes DarkSky's API and makes it more visual. All gesture based. Visualizes cloud cover, too, which I've not seen in other apps. Biggest draw back is there's only a weekly written summary. I like more description.
iOS iphone only. (also on Android)
Free today, apparently
Pretty, so pretty. Interactive. No words (except locations). Fast to fetch the temp data. Data provided by WU! Doesn't tell you what station it's fetching from, though, and you don't know if they use personal weather stations in addition to NWS stations.
iOS Universal (there's also a Mac app and screensaver)
$3.99
This is my bedside clock. Don't use it to check weather. Uses iOS icon badge to show current temp (updates every 15mins) but source of the data is unknown. I use that as a non-precise "oh, it's about 73° now". Interface is "live" satellite clouds superimposed over a photorealistic 3D rendering of the earth, and a live rendering of where the sun's light is falling upon the planet. There are other views, too.
I have this bookmarked to see what happened at my house today.
@briandigital actually, w/r/t the Dark Sky Company's data, it's more than just NWS; check out their Data Sources page for the full list, but here's a handful of them: