Olivier's distinction between live and mix is critical to answering the "best" question. I would distinguish between best piano for "live" performance purposes and best piano for "recorded" purposes, where you're trying to get the most realistic recorded piano sound out there.
The issue has been discussed a lot here (some would say ad nausium) but the landscape continues to change, as do our perceptions.
I have more experience in the recorded piano sound department than in the "live" playing one. In the recorded piano sound department, I have pretty definite preferences. #1 is still the pmi Bos wet or wet/dry sound, and by a long shot. The Steinway B that Olivier mentions was and still is a very good Steinway B, but it always required quite a bit of tweeking to sound like a real recorded piano. The best attempt I've ever heard at making it sound like a real recorded piano, in a mostly solo environment, is Lawrence Lougheed's work. The only other Steinway that seems to me to sound REAL when recorded is the Milan Steinway, which is only available in NI format. Aside from the Milan, the Black Grand Steinway, which I do not own, certainly comes out the best in the demos. It has a more generic sound than the Milan, but should be all rights be incredibly smooth, since it has countless layers, I forget how many. It's a lot.
Ilio's Ivory may be in the running, but until more demo material is put up, it's really hard to tell what the piano sounds like, exactly.
The number of velocity layers is very important. Just as important is the sound or tone of each of the layers. The transitions between the 3 softest layers of the pmi bos are quite audible to my ears, but the tone is pretty much perfect throughout. The vel layers above 64 are accurate, but I wouldn't use them too much, except for f -fff passages. Overuse produces a tinny result. There's a 16 layer version, which is based on the existing layers and overlaps them. That helps to smudge the transition between layers. But I prefer the leaner, untampered-with 8 layer version.
The wet version IS reverberent, which is exactly what you want in a solo recorded classical piano context, but which it seems to me would make the piano less useful in a mix.
Just last night I played the Vintaudio C7 GS3 live at a jam session at a friend's house. GS3 was flawless, and the layers of the C7 sounded great. It could have sounded much better, but I was playing though a single 12" speaker for portability reasons.
The best? Who knows? But I sure had fun playing it in a live context.
I seriously don't think there is a single "best". So many choices, it's a great time to be a keyboard player.
For classical music (in order of my preference):
(1) PMI's "Old Lady" A warm, lyrical, and a spot-on representation of what you hear in a well-recorded Steinway. This is my own current favorite.
(2) SampleTekk's "Black Grand". Serious, dark, and three perspectives to play around with. (3) PMI Grandioso bosendorfer 290.
Jazz/Rock/Live: Others can probably adivse you better than myself.
Aside from these three, I think for true pianophiles there is another must-have piano library: ArtVista's "Malmsjo". While older than the above, the sound of this piano is intimate and inimitable. It is truly a unique sounding library. Hopefully ArtVista will soon have another piano (Steinway B) on the market.
With so many good libraries out there, listen carefully to the demos and let your own ears tell you which library is best suited to you and your music. Any of the above should do you nicely. Good luck and let us know what you decide.
I'm with Jon on this one....I listenned to just about every sample library on the forum, and listenned to a lot of different examples, and for a natural sounding piano, I thought that the Vintaudio C7 sounded the most realistic, and was probably the most balanced sounding piano of the bunch ...It was not brite, but still present enough to cut thru...It was not dark , but had plenty of punch in the lower registers...I didn't find any odd harmonic's or frequency's that stuck out anywhere on the keyboard ... Like I said earlier , very balanced... GigaPulse is just so incredible... It so totally add's to the realizm of the instrument ...The natural resonance allows you to bascially do away with any reverb programs, unless you really want a deep concert hall effect ...Very nicely done...
No 2 ear's are the same, so in a lot of cases, if you like the basic piano that you are listenning to , and like the programming on it, a little tweaking will go a long way...YMMV .....Jim
The piano sample on the new RD700SX is beautiful (I believe it is the same as the SRX-11 expansion board.) A huge improvement on Roland's other pianos.
Of GS3 pianos, I love the Emporer...very rich, individual and full sound. Also Black Grand Close perspective for jazz playing, but I do find the higher frequencies out of balance (too loud)
Have just receieved Vintaudio C7 which I am yet to try.