You should also note Kühni asked for essential releases, which I believe is different from scores.
In other words, you may find that some Golden Age scores are more important than others or so equally important you can't pick, but in terms of CD release, Mutiny on the Bounty certainly is in the top 10 of "the greatest must-have CD releases".
Therefore, I would also include...
The Star Wars Anthology / The 1997 RCA editions of the Star Wars scores; either or both releases; if I can only keep one (list-wise), I might choose the Anthology over the more complete 1997 editions, because it was a major release at the time, and I believe one of the massive ones of that kind (on CD, at least).
The Lord of the Rings; I still haven't gotten the first one, but, whether you deem the whole set of scores as one of the truly best in film music history, or an overrated series of functional scores (I think Shore did a great job), there is no denying this is an exceptional release: a considerable amount of (extra) music, with plenty of alternates to the already available edition, plus a surround DVD version of same score, and extensive liner notes.
Jerry Goldsmith at Twentieth Century Fox. Granted, about two thirds of the scores sampled in there are / have been available singly, in limited and even non-limited form, but this is more than a compilation. This set truly provides a remarkable overview of a good and important portion of the (master)works of one of the legends of film music. And of course, it also features several never-before released scores, some of which may never get another release (at least not before a long time); it was even probably the last chance for such scores as The Detective to get a single note released, since many magnetic tapes were rotten beyond recovery, or barely good enough to sustain one pass.
I'll let you add titles. I can't think / pick any more right now-- especially as CSI:Manhattan is starting right now...
