This is hard, there are so many excellent ones....For favourite preludes i love #20 bk2 in a minor (this is beautiful :)), #21 bk 1 in Bb major and #17 bk 1 in Eb major (just because this was the first P & F i learnt!)For favourite fugues #24 bk 2 in B minor, #20 bk 2 in aminor and #24 bk1 in B minor.I guess my favourite has to be the A minor one by default!!....Its not that its particular hard. I jlove the strident subject in the fugue....short and sweet!
Book2 E major Fugue. This is a hymn tune. Very clever imitations. It is like a 16th century motet and full of suspensions (50 in all). Very moving.
In my opinion, each and every work from the WTC is equally wondrous. There are no duds. All of them are the greatest! We must not single out any in particular! The truly responsible keyboard player must familiarize herself with all 48!