FLAC

FLAC compresses audio the way ZIP compresses a document: the file gets smaller and nothing is discarded. Decode it and you have the original samples back, bit for bit. MP3 and AAC do the opposite — they throw away what the encoder judges you will not miss, and no amount of decoding brings it back.

That makes FLAC the right format for a music collection you mean to keep. If you have ripped a CD or bought an album you intend to hold onto, FLAC preserves the master and can be converted to anything later without a second generation of loss. Going the other way — an MP3 converted to FLAC — makes a large file of exactly the same damaged audio, and is one of the more common wastes of disk space on the internet.

Whether you can hear the difference on a phone with wireless earbuds is a separate question, and the honest answer is usually no: Bluetooth re-encodes everything on the way to the earbud anyway. The case for FLAC is preservation, not playback.

Apple has its own lossless format, ALAC, which it released as open source in 2011. The Music library is built around Apple's formats, and FLAC was never added to the list — not for any technical reason, since the system already carries a FLAC decoder for everything outside that library.

So the practical situation on an iPhone is: the Files app plays your FLAC, third-party music players play your FLAC, and the Music app pretends it does not exist. If you want your collection in Music, converting to ALAC is lossless and reversible — the same audio, a different wrapper.

If you would rather not convert, the answer is a music player that manages its own library. Playlists, artwork, gapless playback and an equalizer all work on the file as it is, and nothing is duplicated.