Robert T. Murphy

This is definitely the most exhaustive commentary on Zechariah up to date. If you have a question about the text or background of Zechariah, this volume answers it. I definitely would not go to this first if you are needing something for a sermon, as it requires reading several pages of dense material to find anything of practical relevance. In my opinion, this is one of the best commentaries on Zechariah, up there with Meyers & Meyers' AYB Commentary.