If you really want to concentrate on your center channel's performance, go with a mono track (a DD-1.0, as the 2.0's send the same signal to each of your mains).
Anything from "Citizen Kane" to "Crimes and Misdemeanors" would be apt. There's a surprisingly good dynamic range in Criterion's recently released "The Cranes are Flying", plus you'll get to see a real good flick. Their "Cries and Whispers" is also an excellent mono track, with all manner of odd sound effects and the full range of human vocalizations (you know... cries and whispers!).
One of the finest mono tracks I've heard (though it might be DD-2.0 - I can't remember offhand) is Fantoma's great DVD of Fassbinder's "Whity". If you're looking for a viewing option rightly labeled "bizarre", then do check this out. An amazing anamorphic transfer and, like I said, an extraordinary mono track.