the schumann is a definite runner up.
im surprised none of the sorabjians have mentioned one of sorabji's quintets yet. ive herd the 1st one, and it was rather interesting. i also hear that the 2nd is a monster.
Sorabji's First Piano Quintet is indeed interesting in the history of music written for that medium, although not one of his very best works. Where did you hear it? It's been performed in NYC (the 1998 world première) and more recently in Europe. The Second Piano Quintet is indeed a "monster" at least in dimensional terms - some three hours' worth, I would imagine, as distinct from the whisker under half an hour of the first; there is now a new typescript edition of the Second by Alexander Abercrombie.
Perhaps some might have expected me to come up with one of these two works earlier in this thread, although you may note that I cited the Schmitt (a work much admired by Sorabji, incidentally, after he heard its UK première about 90 years ago in London).
Best,
Alistair