All suggestions so far are excellent, do look into them.
Another often looked option, that IMO, is still the best sounding acoustic sample available, is Art Vista's Virtual Grand.
http://www.artvista.net/VirtualGrandPiano.html
It's a wonderfully sampled Steinway, and was sampled with the mics a bit further away, so it doesn't suffer from the "plunkiness" of some of the close mic'd samples. You really can hear the wood "cavity" in these samples, and the mid register, which is arguably the hardest to faithfully capture, doesn't suffer from the synthetic sound that many can't seem to over come.
Best of all, is the price.