Went with Octavarium. Not my top favorite, but it's up there, but my reasoning is that because it is so diverse, you get a taste of what DT *CAN* do musically. You have the heavy hitters ("The Root Of All Evil", "Panic Attack", "Sacrificed Sons"), the ballads and straight rockers ("The Answer Lies Within" and "I Walk Beside You"), the hard-rock-influenced songs ("These Walls" and "Never Enough"), and the massive prog-epic ("Octavarium"). The variety, while it turns some fans off (especially after the one-sided, heavy and metal Train Of Thought).
You get songs that sounds like things they've done on every album prior to it, like the U2-esque IWBY, which has been evident in songs like "To Live Forever", "Innocence Faded" and some of the stuff from the 96-97 period before FII. You get a piano-ballad in the vein of "The Spirit Carries On" and "Disappear" (even though TALW isn't half as good as either of them). You get pretty much every sort of DT sound on Octavarium as you would get on pretty much any of the seven albums before it.
Because of that, I nominate it as the definitive DT album.
Oddly enough, my second choice would've been Images And Words, and neither album has an instrumental song on it!
-Marc.