Last year we had a similar discussion that resulted in the "Piano Shootout" that can be found here:
http://www.svpworld.com/tyrospiano.htm

Basically, the idea was to compare the then-new Yamaha Tyros acoustic piano with other digital piano emulations in as fair a manner as possible that would allow listeners to judge for themselves the quality of the piano sounds that were presented. I created three midi files consisting of scale walk-ups at two consistant velocities and a full-keyboard rendition of "Moon River". These midi files were then played on many different keyboards and MP3 files were created and posted so that each keyboard's best acoustic piano sound could be compared directly with the others. Although this was not meant to be a contest, it was very interesting to see that there was no "winner" anyway - practically everyone who listened and offered their opinion had a different preference of what made for the "best" acoustic piano.

If you haven't checked it out yet then it's well worth a listen.