While I’ve dabbled in a range of flashcard apps over the years, I think it’s pretty undeniable that the one I’ve stuck with the best is Readlang . This isn’t because Readlang is the absolute best app — there are a number of points both Anki and Memrise have over it — but really because it’s just so easy to keep adding new words/cards to it. All you’ve gotta do is keep reading, clicking the words you don’t know, and Readlang automatically generates the cards for you. Now a lot of the time, you do have to edit the card a bit (because sometimes it translates it wrong, or else you want to add synonyms, etc.) but most of the time that’s all it takes. You don’t need to brainstorm lists of English words that you want to know how to say them in Spanish (or whatever other language) and then manually create all your cards. Or if you don’t want to do that, then it isn’t like your only option is to be reliant on decks made by other people that might vary wildly in quality or contain idiosyncrasies that don’t work for you. The barrier to entry is just so low and it’s really easy to keep coming back, day after day.
But Readlang does have some problems. You can’t customise the SRS algorithm, cards reset to zero every time you get them wrong and there’s no recognition of leeches (so cards that you keep getting wrong, you’re gonna be seeing them every few days forever), you can’t separate cards where the same Spanish word has different meanings in different contexts (like advirtió being able to mean “warned” or “noticed”), and you also can’t just add words that you want to practise but didn’t see in a Readlang text. Like, writing my review for Harry Potter y las reliquias de la muerte, I had to look up a number of words/phrases that were not in my active recall — sombrío, distópico, presenciar, de un tirón, and even camping, which obviously would cost me nothing to recognise but that didn’t mean I was sure of how to say it or use it in Spanish. To add them back into Readlang though, I think I’d either need to view my own review with the Readlang bookmarklet (which might require Chrome) or copy-paste my review into their site to add it as a private text, then read it from within their interface and click on those words I’d already looked up. Roundabout methods, at any rate.