Options at 70 are certainly far greater in BC then they were in Vanilla WoW. Heroics, in particular, meant all of the colorful and deadly corners of the world had challenging elites ready to be smited by adventurers. Certainly, though, some were better (at least, in my opinion) then others...
Slave Pens - The ultimate quick heroic, with a decent variety of pulls, and no CC required. Learning just how much you could skip was one of the fun parts about this instance. I'll never figure out why there was a colossal lobster monster just hanging around, though.
Shadow Labs - Long, but fairly epic. With a good group, it felt like a real dungeon dive - with bosses and trash that made sense, and a quest line that progressed even in the middle of the dungeon. I think everyone remembers the first time the door to Murmurs room was opened for them.
Arcatraz - One hard, skippable boss, followed by three incredibly humorous, enjoyable ones. Millhouse Manastorm was reason enough to clear the whole place.
Shattered Halls - Love it or hate it, this dungeons provides a place to really stretch your multi-mob tanking skills and grow as a tank. It was grim, tough, and ultimately satisfying to complete. The timer for the heroic version also added an optional sense of urgency.
Black Morass - The design for this place was great. Something different, with no trash pulls/CC to worry about and mobs that came to you! Having read the WoW book series since then, I've also gained an incredible appreciation for this place as a piece of Lore.
Botanica - Lots to kill, with little behind it. Difficulty was right, length was a little on the long side, but not a bad design.
Mech - Borders on too easy. I found the final boss to be somewhat of a let down, for having encountering him all the way from Hellfire to Netherstorm.
Ramparts - Nothing stands out one way or the other. I felt like revisiting the heroic was a shadow of how cool my expansion-day-one first outlands dungeon run there was.
Steamvault - First boss was obnoxious without having one of a few certain classes to deal with adds. Not terrible, but just sort of there.
Sethekk Halls - I didn't run this much, and wasn't fond of it when I did. It only really felt complete if you were running the Raven boss.
Underbog - Long, and filled with a motley of monsters. I kinda feel like this is where all the scraps from Zangamarsh got swept.
Mana Tombs - Escort quests are all ready the bane of groups - putting one that starts in the middle, works backwards, and only yields a reward to a player finishing it the first time was a terrible move. This and Auch. Crypts felt like UB, in that the felt like the entire contents of Terokar just relabeled and repackaged.
Auch. Crypts - Two bosses, and lots of trash adds. The risk to reward ration was just off.
Old Hillsbrad - Another escort, though this one is mandatory. And while the lore part of this dungeon was pretty cool, it was positioned in an odd level range for non-heroic, and was an overly hard heroic.
Blood Furnace - The often forgot heroic. Boss two's gauntlet could stop a mediocre group in its tracks, resulting in a long time commitment for little reward.