Fugue No 2 in C-Minor, BWV 847
The C-Minor Fugue, in three voices, is textbook material for learning what a fugue really is. DaveConservatoire uses a live performance by Kimiko Ishizaka from 2013 to exemplify various fugal constructions in this video:
You can hear the later, studio recording of the same piece in this video. It is interesting to hear how Kimiko's interpretation of the piece evolved from 2013 to 2014, over the course of constant study and multiple performances