I just did heroic MT today. It is definitely the hardest heroic I've experienced. I have 13.6k health and 16k armor unbuffed, but we wiped twice on the first boss and twice on the last boss. The first boss is insane without shadow resistance gear and the last boss requires imba DPS. The second boss sometimes hits extremely hard, but I didn't die. All trash requires good CC.
Other heroics... Botanica is my favorite with a good group. Quite easy.
Blood Furnace: Some mobs hit very hard. The bosses aren't hard though.
SP: I hate the big packs of mobs between the second and the last boss.
Underbog: Not too bad. The humanoids before the second boss are annoying, but otherwise this is easy and fun.
Sethekk halls: If you have good CC, this isn't too hard. Not many heroic badges though.
Shattered halls: I haven't done a full run yet. Only the first boss for Karazhan quest. The big mob packs felt quite tough. I had a feeling that things aren't quite under control.
Arcatraz: I did this with a good group. It wasn't as bad as I expected. I quite enjoyed it. Maybe we had imba group.
Ramparts: Sometimes there are groups of 3 mobs (one humanoid and two dogs) which hit very hard. When I first tried this instance, we wiped several times during the first few mobs. Later I've had better success (I'm in nearly full epics now), but I'm still a bit wary of some mobs.
Steamvaults: If you have decent CC for caster mobs (a mage and a shadow priest are good choice), this is easy. The last boss is easier if there are some melee members in the group since the boss has some anti-spell abilities. I've not tried this instance without good CC.
Still no experience about Auchenai crypts, Mechanar or Shadow labs. I don't like any of those instances in normal mode, although everyone else seems to love Mechanar. I have no experience about CoT instances in heroic mode either.