The Royal Spanish Academy’s dictionary, most likely. While I can easily have conversations in Spanish, the similarities with my mother language every once in a while make me get words confused, specially in conjugation (why does B and V need to sound so similar…). I’ve had it for a while, and although it was a bit pricey, it has saved me quite a few times from awkward mix ups. Also bonus points for working completely offline (looking at you, Yomiwa dictionary…), and having no problems being sideloaded into vanilla Android systems.