SongKong
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SongKong uses its own database sourced from MusicBrainz - which has advantages but it may be out of date by days or weeks.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).
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).