Royal Opera House: Front two row of the stalls, which are at least a bit cheaper than the rest of the stalls (I find being able to see how the singers interact with each other makes a real difference, even if there are some compromises to be made in terms of orchestral balance). If we are seeing a production more than once, we normally try to get the front side seats in the stalls circle which others have mentioned. We sometimes sit in the Amphitheatre, but they can be a tight squeeze and the lack of arm rests means you can be in uncomfortably close proximity to your neighbour.
ENO: we usually try to get an upper circle box as these are the same price as the other upper circle tickets, provide a good view and more comfortable seats, and you can take a few bottles of wine in with you
. I'm afraid that, with the exception of the chairs in the boxes, I find all the seats at the Colisseum to be almost unbearably uncomfortable - I think its because the seats are set too low and therefore don't support my legs or back properly.
Barbican: left hand side of the stalls, about half way back as these provide a good compromise between decent sound, view and price.
Festival Hall: rear stalls, but not under the circle overhang.
Wigmore Hall, stalls, about half way back