This app was mentioned in 9 comments, with an average of 3.00 upvotes
I can't stand Duolingo--no grammar on mobile, useless vocabulary (when am I going to need to say "the elephant drank the milk" in conversation?), annoying gamification features that feel very condescending.
For anyone else who felt similarly, try Lingvist. It's almost the same thing (vocab flash cards) but the pacing and repetition spacing is more natural, you learn words that are frequently used in the language, and new words in flash cards come with tables of declensions, conjugations, and alternate meanings. Also has (minimal) grammar guides outside of the flash cards.
They're adding a paid tier next year and I'm actually looking forward to giving them money to improve it.
Apps and websites are not the best way to learn. But of these apps and websites I've found the best one is Lingvist.
The words are presented in order of frequency, not topic, so you'll learn useful words sooner.
You have to write the word instead of picking from a menu of words, so you have to form proper memories.
The words are always in sentences so you learn context.
The answers are always spoken aloud back to you so you learn pronunciation.
I Just started learning with Michel Thomas German and Lingvist