My choice of "great" (what a nice, obscure adjective) pianists is based not
only on the qualities of the performers themselves but also on the music
they're good at.

To my mind, J. S. Bach is the best keyboard composer, absolutely unrivalled
by any other composer from any other era, although he wrote for the
harpsichord and not for the piano:) If a wonderful pianist is good only at
Liszt, s/he cannot be great in my opinion, solely because of the choice of
music. But if a pianist is good "only" at Bach, s/he can be the greatest by
all means.

So I always separate pianists into two categories:
a) the pianists who are good at Bach
b) the pianists who are good at other composers

The number of good performances of Bach by Glenn Gould is bigger, IMO,
than that of any other pianist.  Gould has enough annoying, unacceptable
(although never dull) performances of Bach but mathematically, counting the
sheer number of good recordings, GG must be the best Bach pianist to me.

For the second category, Donald Satz has given enough names.  I don't
really listen to much non-Bach piano music but I'm currently enjoying
Brendel's Mozart and Beethoven.  Don lists Pollini as one of best in
Beethoven but I have special feeling toward his Chopin preludes: they
were my first classical recording and I'm very grateful to the pianist
for introducting me to the world of classical music.

Juozas Rimas Jr (not the one playing)
http://www.mp3.com/juozasrimas (oboe, piano, strings)