Given the theme "Elevation", Vincenzo Petrali Andante for elevation
http://www.youtube.com/watch?v=Q2mf-PY2heY
Vincenzo Petrali Andante for elevation. Fabio Nava on the Serassi organ of the Basilica of San Martino in Alzano Lombardo. Live recording from a concert.
I wasn't aware of this piece; I'm not sure if it's correct to frame it within this topic (even though you actually opened it) in the sense that a single piece does not make a composer a "great" organist.
To explain myself better, those who were cited made the instrument great and more important (also by evolving its mechanics in some cases).
Zipoli wrote tablatures for the organ, Petrali tested organs with his pieces...etc.
If composers who have written only one piece count.... there are others as well