IMO Bach's too... too convoluted and... artificial. His pieces are almost always polyphonic fugues/sinfonias; even his Italian Concerto is really an extended exercise in polyphony.
Add how he composed his schtuff on a harpsichord and you get this really structured sound. I don't like it at all; I can't add pedal to it, because that completely ruins the tone - however, my whole piano life is just about pedal!
Also, his endings (particularly of his 48 p&fs) are so... sudden. You get two chords and that's it. It's ended. With, say, Beethoven, you get this dramatic overextended coda with god-knows-how-many ending chords, but with Bach, two chords and it's done. Compared to the length of some of his pieces, it's just not... right.
Then with Mozart, his primary compositions were mainly with strings; you don't see many of his piano sonatas out there, do you? What I dislike about that is its inability to create large amounts of notes (contradicting myself about Bach here :V), and when you do have enough instruments to create large amounts of notes (like in Eine Kleine Nachtmusik), it becomes impossible to coordinate! Sound-wise, it's brilliant, but otherwise...
Finally, Beethoven is just pure awesomeness.