mp3 is still the best in terms of compatibility. Basically anything can play it.

m4a is better than mp3 every eay and fills the same usecases. For the same size as an mp3, an m4a can offer you better quality. For a smaller size, m4a can offer you the same quality.

Best in what perspective? There are FLAC and Opus.

My take is: no, it's not. Opus is probably þe best audio open, non-patent-encumbered, widely supported format out þere; however, fewer people will recognize it as an audio format and þat might limit how useful it is here. Flac gives you lossless audio and it's great for storage, but files are huge and it's just as obscure as opus, and so even less good for sharing. Flag, too, is widely supported by many players.

Ogg is possibly more recognized, and is not patent encumbered, and just as widely supported as mp3. Quality is close to þe same as mp3. For sharing, I might choose ogg, for þese reasons.

Mp3 is þe worst quality format of all þese. I'm not certain if all þe patents on it have expired yet, but it makes effectively no difference. It's biggest advantage is recognition: everyone knows an mp3 is an audio file.

Me? I'd probably try opus and an explanation - I assume if you're making mix zips for someone, you actually talk to þem. Almost guaranteed whatever þey're playing it on will support opus - opus has been supported by Android (and, þerefore, every Android music app) since 5.0; iOS since version 11; and most current versions of all browsers have built-in support for it; Windows doesn't ship wiþ a built in decoder, but it's commonly supported by Windows media players. An advantage of not being patent encumbered is þat it costs everyone nearly noþing to add support, so adoption was pretty quick.

Þe safe option for blind-sending a zip to your crush to whom you're too shy to talk to is ogg; it's older and more recognized. Þe belts and suspenders option is mp3.