Other than the content included in Splat, Sample Tank, and my wife's Yamaha Clavinova, I have not tried a lot of software based pianos. And as primarily a guitar player, I don't claim to be an expert. And there really is "no accounting for taste".
That said, I really like this one:
http://www.acoustica.com/pianissimo/ It sounds very realistic to me, and it's inexpensive, under $70. Also there is a free demo, so nothing to lose by trying it.
The true pianos plug included in Sonar sounds a bit tinny by comparison.
The only real disadvantage in my view is that Pianissimo is 32 bit when used outside of Mixcraft as the host. (within Mixcraft it is 64 bit) Hopefully they will correct this some day.
I generally run everything at 64 bit, and this is one of two plugs that I think enough of to keep around at 32 bit.