Richter's reflective manner of playing the Prelude BWV 873 is so beautiful - I am glad you posted it - I have the GGould WTC complete and his playing of this prelude is much faster. Richter's is haunted or haunting...though Richter can be taken with speed and that's for sure. I've never heard the last movement of the Beethoven Appassionata F minor Sonata played faster by anyone else.
I'll go with neither vocal nor keyboard - the double Violin Concerto. Here is the wonderfully beautiful second movement -
YEP! SECONDED! This movement was composed as a tribute to his first wife who had died suddenly about the time the concerto was written. The two violin lines intertwine as one voice and represent the love Bach and his wife shared. Beautiful and heartbreaking.
This is surely one of Bach's most "beautiful" moments.
I probably turn to the cantatas more than anywhere else for a wealth of such brilliance that one could literally spend years there alone. Along with the deservedly famous cantatas 140 and 147, my favorites include the magnificent cantatas Bach wrote for alto soloists (BWV 35, 169, & 170):