Either Sunset or Neon. I'd say Neon though. It's not too bad once you work out the roulette but it's still pretty annoying. At least in Sunset you could just choose the pokemon, albeit rubbish rental ones.
If I ever lose my Gardevoir, Scizor, and Typhlosion at once, my balance is completely thrown off.
I only did it twice, once for the required visit, then again for the Roserade Costume and catalogue update.
Sunset isn't too bad, especially since it's comparable to Emerald's Battle Factory.
neon and sunset are basically based on luck, and for people like me...luck tends to go in favour of my opponent. i do like sunset colloseum though, but i hate neon. but for me it simply has to go to stargazer colloseum, i feel like 492 pokemon have just been insulted to see a parasect ranked on the team of the 2nd to last trainer in the stargazer colloseum, still a tough trainer though as are all of them ... mysterial is just immense, i cant tell you how good it feels when i beat him.