The first movies you should watch are the Extended versions of the Lord of the Rings Trilogy. Since the movies are spread over 2 disks, they were able to up the video bitrates (at least it seems that way to me!) and the DTS-ES sound is PHENOMENAL!
Also, if you are looking for something you haven't seen I highly recommend Appleseed (anime). The visuals will have your jaw on the floor and so far this makes more use of the surrounds than any movie I have seen.
Either way, you will have tons of fun with a good home theater.
