A lot depends on your personal preference - e.g. do you prefer to be close to the stage? Personally I prefer not to be too close to the stage for ballet or a musical, as often you get the impact better from a bit further back. I like the centre of the Parterre best, but most seats in BGE have a reasonably good view. The floor slopes so seats at the back are higher than those in front, so your view is rarely blocked by people in front of you. But avoid being too far to the side (whether downstairs or circle). I went there for a ballet some time ago and was at the side of the circle and didn't have a full view of the stage (even though they were full price dress circle seats, not sold as restricted view.)