It’s really hard to beat Pleco in terms of overall functionality, accessibility (both on iOS and Android), and the sheer number of add-ons that you can get. The variety of dictionaries available for purchase is also a huge asset that makes Pleco invaluable to any Chinese student. However, it is a significant investment to get in all of the features you might want to use (aside from a few dictionary options, pretty much all of the other add-ons cost money. For example, the app has a handwriting recognizing but costs $10 for the enhanced version). One of my favorite add-ons, however, are the Chinese <> Chinese dictionaries, seen in the first entry below.
Find out more about Pleco here.