Bismarck is my favorite for music, I never get tired of the techno that plays especially in phase 2.

For actual challenge I have to go with Ravana though. Bismarck is the same tactic the whole fight, kill adds -> cannons -> dps bismarck -> kill adds -> cannons -> dps bismarck -> phase 2 -> kill adds -> kill more adds -> cannons -> dps bismarck. The final primal fight the OP is mentioning, same thing, the whole fight is just don't stand in things and kill a few adds. We went in blind with a group of 8 players and it was ridiculously easy, hopefully the EX mode will actually be more challenging.

Ravana at least requires you to have more situational awareness, more telegraphs, and different patterns of attack.