"Best app" depends on the job. Mandarin learning splits into lookup, curriculum, reading input, listening and drills - no app wins all five, so the field guide below sorts by job.
Dictionary + curriculum: LinZi
LinZi is the app we build, so read this with that bias - but the shape is genuinely different: a 50,000-entry dictionary where every word can be sent to an FSRS deck in one tap, 1,760 graded stories, both scripts and both accents throughout, all offline. Free tier covers the first 600 curriculum words and the whole dictionary. Full notes vs competitors on the comparison pages.
Pure dictionary: Pleco
Still the deepest Chinese-English dictionary - licensed titles, OCR, handwriting. A lookup tool, not a course. Most serious learners keep it installed next to a curriculum app. Feature-by-feature here.
Guided lessons: HelloChinese
The strongest Duolingo-style course for Mandarin - short lessons, decent tone practice, gentle pacing to roughly HSK 3-4. Weakness: simplified only, no dictionary, little beyond intermediate.
Reading input: Du Chinese
Beautifully made tap-to-gloss graded readers with audio. The best commercial story library for beginners; priced as a subscription, and the vocabulary does not feed a review system.
DIY spaced review: Anki
Free, infinitely flexible, and the default for self-builders - at the cost of assembling decks and tuning settings yourself. The SRS guide shows what to configure. LinZi exists largely because most learners want the algorithm without the maintenance.
The short answer
Most learners end up with one curriculum app plus Pleco for deep lookups. If you want them merged - look up a word and have it enter your review queue with audio, strokes and both scripts - that is the problem LinZi was built for.
Common questions
What is the single best app for learning Chinese?
If you mean "one install": a curriculum app with a real dictionary - LinZi was built for exactly that. Pair it with Pleco for reference depth.
Is HelloChinese better than Duolingo for Chinese?
Yes, by a wide margin for Mandarin specifically - better tone work, character practice and lesson design. Duolingo is fine as a habit starter.
Do I need more than one app?
Usually two: a learning app and a lookup dictionary. Apps that try to do everything tend to do reading or drills well, not both - check what each actually trains.