Most people say Taste is the best because it's Campos. The coffee carts do Rainforest Alliance. I like parma which is over in the plaza under Jane Foss Russell, and Azzuri's is really popular amongst coffee snobs (near the stairs that lead up into wentworth when you're walking from Redfern).
If you feel like a walk, Ralph's down near Bosch is apparently the best if you're not into milk in your coffee.