I've completed 6 fractals so far, that's 2 levels and the Jade Maw boss.
Hands down favorite has to be Freeing the Colossus or Cliffside Giant.
You work your way up a cliff, breaking seals that have the Colossus chained to the cliff. You start off by killing a Legendary boss, taking his hammer and charging the hammer by killing mobs. The hammer comes with condition damage however and your party must coordinate switching off on who is holding the hammer or you die. The hammer must deal the killing blow on the mob to get the seal breaking ability.
The seals need to be weakened before they are broken and you must go back and forth through a back passageway between the seals on each level (ankles, chest, arms), alternately charging the hammer with mob kills and using the seal breaking ability.
On your way up the cliff you deal with gargoyles that will try to knock you down and the flame pops that will blow you off that you need to time right on when to run up.
At the very top you have another Legendary boss and a final seal. The boss spawns adds at each 25% health and will randomly throw a party member into a cage. Break the cage, kill the mobs, charge the hammer, break the seal, keep switching off the hammer. When the boss dies and you break the final seal you get this:
This all sounds pretty straightforward, but words cannot convey how EPIC this fight is. Working your way up that cliff with the Colossus struggling to be free. After he bows to you, if you bow in return you get an achievement.
I completed it at level 1 difficulty, it will be interesting to see how it scales up when like Emeril Lagasse says BAM! and we kick it up a notch and take it to the next fractal level.