Answer #1: The Perfect Piano Practical Answer
The one that works for your situation: fits your song/production, inspires you to create, sounds great live, is in the format you need (Logic EXS, Native Instruments Kontakt 5 or stand alone) etc.
Every situation is different. Some want the perfect piano sample to load fast and to be flexible so that they can work right away. Others want intimate detail and don’t mind if they have to render the performance offline because the piano sample is too big to run in realtime.
Answer #2: The Perfect Piano Sample Technical Answer
Every note would be sampled with 127 velocities for pedal up, down, soft pedal and half-pedalling.
There would be plenty of microphone perspectives including player, hammer, inside (with variations), just outside, room, under and so on.
The recording would be as high a bit rate and sample rate as possible. Let’s dream it to be 32 bit 192 kHz.
Such a piano would be several terabytes in size and there currently is no system that would be able to play it back!
The one that you know. Being familiar with how your piano samples sound is the best way to make them work for you.