The most complete in the App Store
If you are serious about finding both literal and nuanced definitions of nearly every Spanish word, then look no further.
Besides the staggering number of entries and options available to customize the app, I feel the real strength and major selling point is the regular updates and personal responses to questions or complaints is second to none.
I could say many more positives about this app, but will end by saying the only minor shortcoming might be the need for an internet connection for the spoken pronunciations. In my opinion this small inconvenience is far outweighed by the enormity of its data -base.
wallroof about
English Spanish Dictionary, v4.0.0