The title of the "No 1" language learning app can vary depending on different criteria such as user preferences, features offered, effectiveness, and popularity. However, some of the most popular and highly-rated language learning apps include:

Duolingo: Duolingo is one of the most widely used language learning apps globally. It offers interactive lessons in numerous languages, including vocabulary, grammar, reading, writing, and speaking practice. Duolingo's gamified approach and user-friendly interface make it appealing to learners of all levels.

Babbel: Babbel is known for its focus on practical, conversation-based language learning. It offers courses in various languages with lessons designed by language experts. Babbel's structured approach and speech recognition technology help learners improve their speaking and listening skills.

Rosetta Stone: Rosetta Stone is a pioneer in language learning software, offering immersive courses in multiple languages. Its method focuses on immersion and learning through context, without translations. Rosetta Stone's interactive lessons and speech recognition technology are popular among serious language learners.

Memrise: Memrise uses spaced repetition and mnemonic techniques to help users memorize vocabulary and phrases effectively. It offers courses in numerous languages, including user-generated content. Memrise's community-driven approach and diverse content make it a popular choice for language learners.

Pimsleur: Pimsleur offers audio-based language learning courses that focus on listening and speaking skills. Its method emphasizes language acquisition through repetition and spaced intervals. Pimsleur's audio lessons are convenient for learners who prefer to study on the go.
While these are some of the top language learning apps available, the "No 1" app ultimately depends on individual preferences, learning goals, and the specific features that best suit each learner's needs. It's recommended to explore different apps, try out free trials or sample lessons, and choose the one that aligns with your learning style and objectives.