J S Bach was the 1st classical music I ever liked, and he is still my favourite composer. Since then I learned that I like Baroque music quasi-exclusively, Late Baroque (Bach, Rameau, Handel) in particular. Mozart, Chopin, etc sound like elevator music to me. Beethoven is sometimes interesting but not always. This is why I'm a student of the harpsichord at the Conservatory and not piano - the répertoire I need to play is all Baroque!
What I like in Bach is the near-inevitability of his mathematical precision, incredible variety and original genius of each piece of music, and especially his perfect counterpoint in 2, 3, sometimes even 5 voices on a single instrument
I love playing and listening to different, complete and perfect melodies with just my two hands. It feels like my brain expands to follow two different threads of music, forgetting all else. It's the only time I can "live in the moment" and truly rest.
I'll stop raving about Bach now 
My DC love Baroque music, incidentally, especially Bach's pieces from the Notebook of Anna Magdalena Bach. We have the CD in the car and they often ask to listen to it. DS (8) has been going through it this year, choosing the pieces he wants to play. His favourite is still a bit too difficult to him as it requires a hand span that is out of his reach for the moment, though.