You will get a more accurate response if you include the type of music you play. Or if you can indicate songs that feature the kind of piano sound you are looking for.

Good Luck

i'd say there is no "best", but there are some very good ones.
check native instruments akoustik piano or steinberg the grand. they are really good. another one would be pianoteq. it's not a sample piano but a piano modeller. i've listened to some examples on their website, and it sound quite good, too. there's also a trial version for download.
if you want a good piano without paying hundreds of dollars then download the free edition of sampletank 2 with the hq piano. it's probably the best piano you'll get for free. i really like it.

look at the websites of the manufacturers. there are sound clips you can listen to and decide for yourself, which one suites you best. it's often not only a matter of sound quality but also of emotional quality. especially with pianos.

the "best" is probably the most accurate one (about layering, quality of the recording equipement/environment/sound engineers and so on).
Just for fun I made on my site a comparative test between Akoustik Piano (350$) and HQ Piano (Sampletank2, Free...) using exactly the same MIDI (a Sonata, by Mozart), and the results are to me interesting (given you will listen with a good pair of headphones in a silent room...) :)

IMO(humble or otherwise) the genre you are playing has nothing to do with the piano sample...(barring special effects like"honkytonk" or something similar) akoustic and the grand are both well respected... the pianotek also mentioned is kinda interesting and takes far less hard drive as it's not really sampling... however far and away the best sampled piano is IVORY.... this is discussed ad nauseum at the keyboard mag site....

I am listening to theyr sample and indeed it is astonishing...

ivory!!! fantastic!! i have no words