Depends what you prefer - some would say the closest area in front of the 'stage' but that's all flat (ie. no tiering). I'd go for a few rows up, slightly to the side (if central, you're furthest from the performers), but it's all personal taste. Avoid the gods!
The hall was designed to give the wealthiest people the best seats. That means that the boxes (Grand Tier, Second Tier and Loggia) are among the best places. I've enjoyed sitting at the front of the Circle (immediately above the boxes but below the Gallery). Otherwise the front of the stalls (particularly in zones J or L) or the centre of the Arena (when it's used for seating) would be well worth considering:
http://www.royalalber...ing-plan/default.aspx