It's probably easier to understand if you actually like the album, but, to keep it simple, it's Brian Wilson's innovative production and arrangements which give it its status, I think.
Personally, I also love the tunes, the voices (Brian in particular), and, of course, the harmonies.
Now I've talked you into seeing things my way, try Sunflower (no. 380): the great unsung (until this poll, I suppose) album in their canon.
Were the contributors asked to give their favourites or the ones they though were 'the best'?