To be honest - chest and calves. I find that my shoulders and tri's grow much easier than my chest because they get their own workout plus the secondary effects from my chest workouts. I'm currently hammering calves so let's see how I go over the next year. Third place would probably be biceps.
My guess is that I'm good at back exercises, for example, or I'm good at the bang for your buck back exercises (like pull ups) so that's why my back has been growing so much in the last year, compared to say chest.