I've only tried two dictionaries, the American Heritage Desk and The American Heritage 4th Edition. The Desk has fewer entries and more concise but less detailed definitions, I moved on to the American Heritage 4th Edition dictionary, and it's served me well. I sometimes read books on neuroscience or discussions between Buddhists and scientists on the brain, and the terminology the people in those fields use are not common. The American Heritage 4th edition has never failed to have the entry or to define the word well.
The other dictionary that also has extensive entries is Merriam Websters (they have a few dictionaries, so if you're looking for extensive entries, check the app description carefully to make sure you're getting the right one).