I prefer the Pleco platform. (www.pleco.com) It may be overkill for some, but I frequently use it to reference multiple dictionaries on my iPhone. They also have a great OCR (optical character recognition) system built in that I find helpful for quickly deciphering Hanzi. The dictionaries also have text to speech and stroke order diagrams built in.

有道词典还可以………

there's no "the best" yet