I've had my best experiences in the first few rows of lower balcony,
too. I would say the very first row except I hate having my knees and
toes pressed up against the concrete wall at the edge of the balcony. I
guess there was enough room, but from a pyschological perspective, I
felt trapped.
My next choice would probably be the gallery, but it just feels too far
away (last time was Osmo Vanska conducting a Sibelius symphony). The
terrace behind the stage is an interesting experience, though it was
rather loud and bright (in this case, music by Elliot Carter). The few
times I have been on the main floor I have not been pleased. In the
center about halfway back the sound came across as slightly dull (a
Mitsuko Uchida piano recital), and near the stage to the right sounded
mudddy and boomy (Barenboim conducting Brahms' Symphony No. 1).
I've never been in any of the box seats. I've heard that the sound
there isn't that great, but I have no idea if it's true.