It probably depends on which language you're learning and whether there's a good deck available but I've moved onto flashcards with Anki.
The main problem with Duo (before all of its current problems) used to be that it didn't really give much context unless you were on the full website and clicked through to read the grammar rules, but the card deck I'm using for basic Spanish has all of that built in and visible from the start so it's been a good foundation on the language.
I've also heard excellent things about something called the "pimsleur" method but that seems expensive to access through official means. I'm sure there are other ways to try it out, of course...