Favorite piece of Bach’s? That’s like asking for my favorite child… well, if I had more than one kid, that is. 
I’ll compromise and write down the first three that came to my head:
"Great" Fugue in G-Minor, BWV 542, really the E. Power Biggs version that was played on the Flentrop organ @ Harvard. I just love this damn thing, even though it’s constantly overlooked in any discussions of Bach and/or his organ works (though the Fantasia is more-referenced, I don’t like it as much).
Brandenburg Concerto #2 in F-Major, BWV 1047. Yeah, it’s a cliche, but if you wanted originality you shouldn’t be asking me this question. 
"Little" Fugue in G-Minor, BWV 578. So, I was in “Appreciation of Music 101” and fell asleep. Waking up a few minutes later, I was hearing the most FREAKING AWESOME PIECE OF MUSIC I’VE HEARD, LIKE, EVAH! I mean, really, it was that great - I couldn’t believe what I was hearing, it was like a completely new way of looking at the same 12 notes. And I don’t think it would have had that sort of affect on me had I heard it wide awake - there was a particularly other-worldly/dreamy quality about the song that made it go #1 with a bullet in my heart. Again: E Power Biggs on the Flentrop organ @ Harvard.
That experience changed my life: that very day, I started buying Bach music. Within a few weeks, I had determined (with all the certainty that being 21 (I was late taking this particular elective) brings to a guy) that all modern music sucked, that Bach, etc, truly ruled the musical spheres. I completely missed the Grunge scene. Totally missed out on 99% of music made in the 90s and the majority of my music purchases over the next decade were of the 3 B’s variety (well, 2 B’s and an M - I was never too big a fan of the romantics, preferring Bach, Beethoven, Mozart, and their contemporaries. Honestly, it’s as simple as the fact that I kept turning my volume up and down, which irritated me to no end - I want to put it at a level and hear the damn thing, not be blasted out by fuckin’ trumpets after nearly being lulled to sleep during a soft flute passage - WTF? But I digress…
)
Anyway, 2 organ pieces and a Brandenburg. That’s what I got.
Happy Birthday, you old wig!