gregjammer
Feather Trainer
In my opinion, courtyard round 2 was the hardest, because the 2nd quarter and the 2nd half of the 100 trainers you face are tough, and one unlucky moment on the dart wheel may not heal you when you most desperately need a healthy team to survive, and will almost definitely put a loss on your record that will force you all the way back down to trainer no. 1 should you challenge courtyard again!
The second-most hardest colosseum has to be sunset and neon, but because they are both easy in round 1 and luck is not that necessary in that particular round, and since they have a roulette or random pokemon selection from a fixed set that doesn't necessarily cover all 490 pokemon usable pokemon at the time, I voted courtyard because it combines Neon in some ways, AND the computer randomly selects the roster of your opponent before the battle starts...
The second-most hardest colosseum has to be sunset and neon, but because they are both easy in round 1 and luck is not that necessary in that particular round, and since they have a roulette or random pokemon selection from a fixed set that doesn't necessarily cover all 490 pokemon usable pokemon at the time, I voted courtyard because it combines Neon in some ways, AND the computer randomly selects the roster of your opponent before the battle starts...