If I want to ask what concert/opera/ballet is playing would: "Который спектакль идёт?" be correct or is there a better phrase that is used more commonly?
Который is wrong in this case. You may only say Какой.
Also talking about "classical shows", like classical music concert, opera or ballet, there's a tradition to use the verb "давать" instead of "идти".
E.g. What ballet is playing in Bolshoi = Какой балет дают в Большом? Although nowadays many people say "идёт" too.