I agree that Musicbrainz Picard is probably the best option to tag your files en masse. But I always caution that the online sources (Musicbrainz, Last.FM, Discogs, etc.) are full of errors, so the tags probably won’t be perfect. Depending on your tolerance for those errors, you may want to start with mass-tagging, then follow up with a tool to manually fix those errors. For that, I recommend MP3Tag (for Windows or Mac). Picard isn’t well suited to editing tags in individual tracks.
Once your embedded tags are in good shape, you’ll want to make sure Plex is properly configured to use them. It’s more than the one checkbox.
If we’re talking about classical music, you’ll find that Plex has some shortcomings. Plex only makes use of a small sub-set of the tags that can be in your files, and “Composer” isn’t one of them. My solution has been to use the composer as “Album Artist.” There is a feature request to improve this, but it has been a long time coming.
