Although Anki runs a close second here, for a beginner clawing their way to intermediate this app is the Vocabulary Master Trainer! Ill use Anki for custom list (particles etc.), but for second to none well made flash cards with native audio and EASY to use Anki/SRS goodness, this app wins hands down. Easily worth the minor subscription fee. Ill happily pay to have Iknow curate and expand this app. Also, check out the Japan Times Genki apps as well, those are excellent. Especially if youre studying using Genki textbooks.