Hi, David. Would you care to define what you mean by "greatest"? Most popular? Most often performed? Most satisfying to sing? Highest quality? (Oh yeah, there are no absolute measures of quality. A pity, but still quite true.)
Miriam's Siegesgesang for soprano solo, SATB chorus, and piano (the biblical story presented in a poem by Franz Grillparzer). This is a mulit-movement, cyclic piece that has some of Schubert's most inspired tone-painting, romantic harmonies, and his best fugue. It was written in the last months of Schubert's life and he probably intended to orchestrate it. It was later orchestrated by - I think - Franz Liszt, but it works great with piano. This is a very aproachable major work that a good high school, college, or community choir could perform well
Second place: Die Nacht for tenor solo, TTBB chorus, and piano.
I find this fascinating! There ar epieces mentioend here I woudl never ever put on a great Schubert works like and pieces I would. Isn't that just exactly what makes our choral world go 'round and makes our programs so varied and interesting!
I can second Steven Edwards' nomination of Miriam's Siegesgesang. It is absolutely gorgeous--and when sung with incredible musical sensitivity, attention to every rich detail Schubert left on the page, with incredible soloist, it is a real gem. I also encourage you to look at Schubert's Des Tages Weihe "Schicksalslenker." The opening Baritone Solo is some of the greatest music ever composed by man.
Since no one has mentioned any of the exquisite male chorus settings here (e.g., Nachthelle, Ständchen, Gondelfahrer, etc.), let me throw one not very well known work in the ring.
There are two settings of "Gesang der Geister über den Wassern" for male chorus; the more extended setting for TTTTBBBB and 2 violas, 2 cellos and double bass (D. 714) is an amazing work, about 10' duration. There is a wonderful recording of it on YouTube by the Norwegian Soloists and Camerata male voices under Grete Pedersen.