If an open source music format isn't a requirement for you, AAC is as efficient as Opus and playback support is universal – no version of iOS or Android has ever shipped without AAC decode support, for example.

Mp3tag also handles MP4/M4A files wonderfully.

AAC when well encoded sounds great, but there are too many sub-par encoder implementations that just don't sound good - sometimes worse than MP3.

If you're encoding AAC use either Apples encoder, or one of those from Fraunhofer:- Fhg-AAC available as a paid plug-in, or free with WinAMP, or the Open Source FDKAAC implementation originally for Android, but now available cross platform.

> …there are too many sub-par encoder implementations that just don't sound good - sometimes worse than MP3.

Whoa, I'd never imagined that could be the case and appreciate the heads-up. I've only had experience with the Apple and Fraunhofer encoders so far.

Mind calling out the offender(s)? This summer I'm releasing a best practices guide (and hopefully tools) to help podcasters move to AAC, and that would be useful to mention.

The FFMpeg wiki has some information, including calling out its own "vorbis" encoder as sub-par and not recommended (but apparently they also support a different "libvorbis" encoder that does not suffer from problems?) See https://trac.ffmpeg.org/wiki/Encode/HighQualityAudio

Also for AAC in particular there are a few options https://trac.ffmpeg.org/wiki/Encode/AAC

One workaround is to put your Opus audio in either a webm or mka container.