Friday afternoon, Blizzard employee Bornakk posted a poll on the General boards to find our favorite 5-man dungeon of the Burning Crusade. As you can see in the above image, the Caverns of Time is rocking the charts.
I can't say I disagree with the numbers there, they're pretty close to my opinions as well. Old Hillsbrad and the Black Morass offer something that most of the other instances don't: Fast pacing, minimal trash pulls, and a story element. The instances aren't very long, either. Black Morass is what, thirty minutes? These dungeons on Heroic remain difficult and dangerous, adding some new elements to the instance, such as a necessity for speed and stealth when it comes to the patrols in Old Hillsbrad. Some of the other Heroics do this as well, but most of them coast along on 'ow this mob hurts a lot'.