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I have tried out at least two dozen language-learning apps over the past several years, and I think it helps to first know what kind of learner you are (visual, auditory, etc.) and how you most like to interact with your device.
For example, I listen to a lot of playlists and podcasts on my phone and I know I'm easily addicted to silly mobile games, which means I gravitate toward things I can listen to or play with.
If you're looking for an app, I'd start with bigger-name and higher-rated apps like Duolingo, Babbel or Rosetta Stone. All three of these options are available for iOS and Android devices, and all three incorporate visual, auditory and written learning to keep you actively involved. Duolingo uses gamification to encourage you to use the app every day, which is pretty effective; Rosetta Stone has nicely organized step-by-step learning; and Babbel uses 'conversations' to guide you through courses.
A few that I haven't used yet, but am curious to try, are Memrise and Drops. The downside to these, as with most language learning apps, is that there will almost inevitably be a subscription wall at some point, meaning you'll have to shell out some cash to get full access.
Once you have some familiarity with the language you're learning, consider downloading an app that's encourages you to practice and use the language. Busuu, HelloTalk and Mondly all teach by having you interact with others (usually over chat) who either speak the language natively or are also practicing.
Because immersing yourself in the language you're trying to pick up can be helpful, you might also want to think beyond learning apps. I've used flashcard apps to brush up on vocabulary words, downloaded podcasts in Spanish and set up watchlists of telenovelas on Hulu. I've even made Spotify playlists (some apps, like Learn Spanish with Lirica instruct by teaching you the words to popular songs) and toggled the settings on movies and TV shows to show the Spanish-language version (The Incredibles was a particularly fun choice). If you're brave, and you know your phone's settings well enough, you could even turn your phone's OS to a different language as well. ¡Buena suerte!