For most iPhone users, AAC is the best everyday audio format. It is efficient, sounds good at practical bitrates, and is deeply supported across Apple devices and services.
If you want lossless audio inside the Apple ecosystem, choose ALAC. If you need files that work almost anywhere, MP3 is still a safe fallback.
AAC is the best default for iPhone music files, downloads, and everyday listening.
ALAC keeps all audio data while fitting naturally into Apple workflows.
MP3 is less efficient than AAC, but it remains the broadest compatibility choice.
FLAC is a great lossless format, but Apple users often have an easier time with ALAC. FLAC can be played with supporting apps, but ALAC is usually simpler for Apple Music-style libraries.
Bluetooth listening changes the practical answer. Even if your source file is lossless, wireless playback may involve additional compression. On iPhone and iPad, AAC is often the practical Bluetooth codec, which is why wireless Apple listening is closely tied to AAC support. For more detail, see Bluetooth audio codecs explained.
For context, read Audio quality explained, especially the sections on human hearing, environment, and playback equipment.
Use AAC for normal iPhone listening. Use ALAC if you are building a lossless Apple library. Keep MP3 only when universal compatibility matters.