Based on Izzy's suggested list and some searching I have field tested the following apps over a 3 week period and found a clear winner:
Bravolol Spanish English dictionary. Simple and quick interface. It has a splash screen but you can leave it open, so it's there when you need it; you can immediately start typing*. It shows related terms (handy), shows if it's a noun or a verb (also handy). It is automatically bilingual which saves time, and a very important time saver: it finds jamón if you type jamon.
Kreatorz Spanish English dictionary. If you browse through the words it takes several seconds to look up, very strange. Immediate search is quick though. Then again, the back button is slow, so impatiently you tap again and then you are out of the program.
When connected to the internet the occasional full screen ads are ugly. Does not find jamón if you type jamon.
Prodict Spanish - English dictionary does not find jamón in the immediate search (dropdown) but will show it in the similar list (all words starting with jam); has some unnecessary settings (text size?); no fast switch between es-en and en-es (you have to go into settings).
Its bells and whistles like favorites and history may be useful for some but not for me - looking the word up again is just as fast. The same for text translation: this works well, but when do you want to type in entire sentences?
(BTW this feature does not work when you are offline, which would explain the good quality: it uses an external service, probably Google translate).
Free Dict Spanish English does not work in airplane mode, which immediately disqualifies it.
Note: I have not tested the quality of the pronunciations because those were not important to me.
* The only downside I found is that you have to backspace your previous search