As someone who used to work there, I agree with cinders- it depends on what you're going to see. If it's something visual(e.g. opera or ballet) don't go for the side balconies (red and green) as you'll only get to see half of what is going on. If you're going to see an orchestra, the choir is best as you get to see exactly what's going on with the conductor. Generally my preference would be for the yellow balcony, somewhere in the middle of an aisle- though just like cinders said if you've a problem with heights stay away from the front row- lost count of the amount of people who had panic attacks from sitting there!