Until recently, the two front runners in the virtual piano race were TruePianos and Modartt PianoTeq. TruePianos was slightly more affordable and was generally thought to be more playable and realistic, while Pianoteq, although an impressively flexible and powerful piano model, suffered from artificial overtones. Since Version 3 of Pianoteq, this is no longer true, in my opinion. Pianoteq has seen some impressive upgrades and options and is definitely worth the additional expense.
However, I've stuck with TruePianos. It's still a very inspirational virtual instrument.