When using superlatives (the best, the worst, the tallest, etc), we must use "de" not "en". In this case, "de + el" = "del". This doesn't work when translating directly from English so this is something you must know in advance. Duo should add this in their unit description (which you can only see when using the phone app). With comparative (better, worse, taller, etc), there is no need for this.
which you can only see when using the phone app)...
They USED TO have it on the web app as well. Why would they have it on one type of app but not the other?
I had assumed the web app was there first priority for updates. Maybe it will soon be taken away from the phone app when it recieves a major update.
Also, "café" and "coffee" are two different things. One is a shop, the other is a drink.
The Spanish sentence is using a different word order, a different grammar than your translation. In the Spanish sentence, cuál is on it's own and "el mejor café" is a unit, but you moved the coffee shop to the question word.
- ¿Cuál es el mejor café? - Which (one) is the best café?
- ¿Qué café es el mejor? - Which café is the best?
Saying "the best one" is not really wrong, but uncommon.