VLC on Android supports opus. Unfortunately, most other players don't (I think this is a actually a bug in the Android media scanning library and not the players themselves though)
Mp3tag also handles MP4/M4A files wonderfully.
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.
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.
Also for AAC in particular there are a few options https://trac.ffmpeg.org/wiki/Encode/AAC