I voted for The Astonishing, mostly because in the 3 years since its release, I don't think I've listened to it more than a few dozen times. At least with WDADU, since I discovered DT, I've probably heard it more than a dew dozen times, and to be honest, I would rather listen to the debut than TA anyday.
Just above those two, though, are probably FII and ACOS. Not that they're bad albums, but I don't revisit them as often as the albums that come after it. In fact, anything prior to SFAM doesn't get a lot of revisits lately, as much as I like IAW and Awake, but I've listened to those albums and songs so much that I don't find myself reaching for them as much as I used to, back when I first got into DT.
This whole thread has given me the itch to go back and re-listen to all of my DT albums, though, so I may end up diving through the whole discography again sometime soon, especially given how great d/t has been so far!
-Marc.