Now, Blizzard hasn't actually said that they're going to go back and create a Heroic mode for all the old instances. But I think we can all agree that they really should. Of course it's asking a lot for them to both tune up old instances and recreate all the loot tables, but they don't have to do that-- they can either create one big loot table for all the bosses, or just create a rep turn-in system (just like the current Heroic instances have).
So let's assume, just for the sake of argument, that Blizzard does decide to go back and put Heroic mode on some old instances for the next content patch. The question is: which one first?
Zabba says Blackrock Depths, but as someone who has already spent too much time there, I winced when I first heard that-- BRD is neat, but takes long enough as it is. Lots of players say that Blizzard should start at the beginning and make a Heroic Deadmines, and I have to agree that it would be a lot of fun. Personally, I'd love to run a Heroic Scarlet Monastery, and considering that's one of the most popular instances in the game, that's the one I can see most happening. A Heroic Stratholme would also be a lot of fun for sure as well.
And which one would be the worst? While we all seem to agree that Blizzard needs to do something with the old 20 and 40 man raid content, I'm not sure making them Heroic is the best solution-- would you really want to go through the chaos of a Heroic instance with 20 or even 40 people? Making them Heroic 10 mans is a nice option though. And as for worst, I have to agree with what a few players said: Heroic Uldaman would just be ugly. Or Heroic RFC-- can you imagine wiping when Taragaman the Hungerer crits your MT twice for 10K each time?