SongKong. While not absolutely essential (you can just use Picard), SongKong is useful for tagging larger number of files and picking up the MusicBrainz Ids, prior to any fine-tuning with Picard. Cost is £25 from http://www.jthink.net/songkong/.
SongKong uses its own database sourced from MusicBrainz - which has advantages but it may be out of date by days or weeks. Also Picard has its own tag scripting tool (and my plugin); if you don't use Picard then you will need something like Mp3tag if you need to extend the capability of SongKong (a scripting tool is promised for SongKong).
Read the manual - it is very clear. Pay particular attention to the "Classical" settings. - the settings I use are shown below:
SongKong adds a special "is_classical" tag based on tables of composers etc., which can be very useful as Picard/MusicBrainz does not do this (although it can be set as an option in my Classical Extras plugin, SongKong has a more sophisticated database for determining if music is classical).