When you find that perfect piano, please report back!
 
Also let us know if you run across any unicorns during your search. 
 
I've got a bunch of piano libraries, but selecting the best one for a given application always entails some trial-and-error. My favorite these days is Keyscape. It's gorgeous. Wonderful for jazzy stuff, classical music or pop ballads - but I wouldn't want to crowbar it into a Lady Madonna cover. If rock 'n roll is your focus, look at sampled Yamaha C7s (there are many) or uprights.

Pianoteq Stage is only $99.  I'm not sure if it a perfect piano, but it looks perfectly priced! 
 
https://www.pianoteq.com/pianoteq_stage
 

Pianoteq Stage 6 is a popular choice for the budget-minded musician. It is based on the award winning physical model, offered in Pianoteq 6 Standard and PRO, praised by many musicians for its close intimacy and responsiveness.
Pianoteq Stage 6 includes the same instruments, sounds and playability. All this for an unbeatable price.

 

Pianoteq Stage can at any time be upgraded to Pianoteq Standard or Pianoteq PRO to gain further sophisticated features, such as to tweak the physical model, to load Scala temperament files or to position virtual microphones around the instrument. With Pianoteq Stage, you are not locked to a certain product but have the option to upgrade whenever needed.

Pianoteq Stage supports all Pianoteq instruments, and if you upgrade to Pianoteq Standard or PRO, the instruments themselves upgrade automatically at no additional cost.

Pianoteq Stage supports all Pianoteq instruments:

    Includes 2 instrument packs
    Free download of our KIViR collection of historical instruments and Bells
    Optional instruments are sold separately.

Keyscape’s C7 is lovely, but not cheap as you’ll need to pay for the entire package. But it’s definitely worth it. The package I mean.
I do believe the ImpactSoundworks C7 is at least as good. Same goes for the ProductionVoices one.
Talking about unicorns, apart from mighty sweet OrangeTreeSamples Rosewood which should get an imminent update, there’s still that interesting TrueKeys Ravenscroft and the all-singing Wavesfactory Mercury.
Then, if you get serious, think Bechstein Digital. It’s the thinking man’s Bechstein from the C. Bechstein Manufaktur itself and allows for some serious sound design.
But now, lo and behold, the amazing Embertone Walker 1955 has landed. It’s the best sampled Steinway I know and it’s incredibly versatile. Furthermore, a very capable Lite version starts at $30-ish. Next in line, as far as cinematic grands are concerned, try the CineSamples CinePiano. But the Walker can do cinematic too, if you pick your mics right (in the full version).
Finally, even though sampled grands have become quite playable, it’s piano modeling by Pianoteq that takes the crown. Just demo their Blüthner and their latest Steingraeber. You’ll know why.
Conclusion? Get a sampled Grand to your liking as well as a Pianoteq favorite. Chances are you won’t leave your home for a while.

The Indian Rhino
Rhinoceros unicornis
 

The OTS piano is a C7, and currently part of a group buy that's already at a 30% discount and easily on its way to 40%. If the group buy keeps growing at this rate, the Rosewood will ultimately be $71.60. A whole lot less than Keyscape, that's for sure. Although with Keyscape you get some really nice EPs and eclectic vintage instruments as well, and if you also have Omnisphere it'll expand that instrument's already-monstrous palette even further.

Keyscape could be a consideration if the Lite install was a separate offer. But not for $400. And no demo.

It seems most everyone believes there is really no alternative to a real live analogue piano and I tend to agree. Being a pianist, I've tried so many electronic versions I've have to write a spreadsheet with all their pro's and con's so I can go back and see what I've liked and not liked about each one.
I've not found one software piano that has the touch variations of a real piano (not by a long shot).
Maybe you can set up your electronic piano to give you the sound you want for any one particular song, but then you'll have to change that for the next song, or even passage of the same song.
I think, most every software piano has it's advantages over another, and it really is a matter of choosing the right one for the music you are recording.
My go-to pianos for use with Sonar are True Pianos and Addictive Keys, but this is mostly because I'm familiar with them and can get the sounds I need without too much grief. And True Pianos have a very small memory footprint, which is an issue for my 32bit systems.
I have four (read four) hardware keyboards. Two Roland's, a Yamaha, and an Ensoniq. I find these sometimes provide more realness in regards to playability, i.e. they have better touch control than pure software keyboards. But it's still 'horses for courses', that's why I have four.
If I want a real piano sound then I use the Feurich analogue I have in the studio. It's no Bosendorfer, but it's strides ahead of any electronic alternative I have, or have ever tried.
If you want a perfect piano, and you have the space, buy a real analogue piano. You won't be disappointed.

My piano has a nice big gouge along one side as a souvenir of its last move. It's staying with the house when I sell it. A solid hunk of wood and metal, it takes four people just to slide it across the living room. But if there's ever an earthquake I'm diving beneath it.

Duck, and cover.

I think its to the point now where talking about the differences between particular piano vst's and/or real pianos is similar to guitar players talking about how they only use guitar bodies made from certain woods because of the 'tone' that wood produces.

Does one wood sound different from another? Yep. But you and maybe ten other people on the planet are the only ones who will ever notice.