What for me has been really interesting is the Cantatas.
I listen every Sunday to the Cantatas for the day, and have been doing for over 20 years, so from listening point of view, I know them pretty well, though not really as a musician: I'm not really able to devine the structure that underlays them, or understand compositional technique, though I can follow the score if I have it. My own collection is based on the Teldec Harnoncourt Leonhardt set, with a sprinkling of other recordings. On cassette I've got quite a few of the first Rilling set, and recently I've picked up some of the new Gardiner recordings from the Pilgrimage and some of the Suzukis.
Hearing the Canatas intensively over the last couple of days - every hour or so, on avaerage - and in a wide range of versions, going from some early 1950s recordings to the latest Suzukis and Gardiners has been challenging to some of my preferences, but I must say, maybe if its only through the familiarity I have with them, I still find the Teldec set suits me best. On Saturday they talked about different performance styles and the ongoing sets. All very interesting!