Well, its hard to say what kind of piano vst is the best, due to the sound you want ect, its the same in real life, a Steinway dosent sound the same as a Bosendorfer or a Yamaha... no brand of pianos sounds the same. So its all in what you want.
I use about 10 different piano vst's and its all in what genre your creating, and what synths, bass, ect ect you are using.

Some good ones:
Pianissimo
Ivory II Grand Pianos
EastWest/Quantum Leap Pianos
Fazioli

Also there are some great ones from NI:
Alicia's Keys
Garritan Steinway Piano
Braunschweig Upright Piano