Comment by retrodaredevil

The cool thing about MusicBrainz Picard is that it puts not only an ISRC metadata tag on each file, but also a UUID for the MusicBrainz recording and for the MusicBrainz album id.

The idea here is that if you use something like MusicBrainz, you can actually retag all of your files in bulk if necessary because MusicBrainz Picard knows exactly what release each file belongs to. You can then configure MusicBrainz Picard to tag your files to your liking. It's a really great piece of software.

If you are tagging files manually, I think an ISRC tag is the bare minimum because it can allowed automations like MusicBrainz Picard to easily identify what each file is.

As for what version of ID3 or ID2, I'm not sure. It might depend on the software you use to play the audio files. The reason I personally use MusicBrainz Picard is because its MusicBrainz specific metadata is read directly by Plex, so even if the other metadata on the file is bad for some reason, Plex will match the MusicBrainz tagging with the correct release. I mean, Plex uses MusicBrainz internally for its metadata, so it's a safe bet for my purposes.