I have found Dark Sky is great in populated areas (NYC, Denver, Houston), but Wunderground seems to do better outside of the metros (Lower Hudson River Valley, Catskills, Adirondacks -- you can probably tell where I live and work, now).
I've found the best data comes from the nearest/best doppler radar(s). Often this is one of the local tv broadcast stations. Obviously this radar data is licensed to various weather service providers but are usually owned by a broadcast television network operator.
I really like Weather Undergrounds local stations. Being able to see that a location at the top of a hill in town is a couple degrees cooler or windier than one down the street has proven to be really useful when decided to bring a jacket or not. That said Dark Sky's UI is top notch and the data seems to be pretty solid as well.
Where I live (Oslo, Norway), the predictions of Dark Sky and Weather Underground are both less accurate than just making an uneducated guess based on how the weather has been developing the last couple of days. I guess they work better in North America
In my part of North America, you can do better than any weather forecasts that I've ever seen by simply predicting that tomorrow's weather will be pretty much the same as today's. And all forecasts beyond three days are essentially worthless.
