lapieuvre
You have to look at Quantum Leap Pianos.  They are on special these days. 

+1  Take a look and listen at the EastWest site.  Not cheap, but not the most expensive either.  If you're having issues with all the "included" piano plugins you probably have a refined ear.  With QL Pianos, you have your choice of loading any of 4 beautifully sampled pianos... or all four if you have the space.
 
Best,
Rob

Be careful with EastWest's Piano's, you need tons of Ram, but they are excellent. EastWest also has a great Steinway and Bosendorfer in their Goliath VST, which I what I use for all of my piano sounds.

EAST WEST = needs a dongle 

You will never regret buying Ivory II.  Ivory II Grands is fantastic, and if you can live with one piano, get their American D.
 
I know I said this above but I feel very strongly that it is as good as anything out there.

REALLY expensive though Konrad - for not a lot of gain over the lower priced options imo - and you need a DONGLE  !!!!

Dimension Pro's piano samples sound very clunky and you need to use notch fitering to get rid of horrible resonances and mechanical sounds.  But despite this they seem to sound great in a mix, so I use them a lot.
 

Have you tried mixing 2 pianos? I like to use the true pianos and dimension pro 9ft grand I think. Spread a little, add some breverb. Also you can mess with the options etc in true pianos.

I agree with konradh who had suggested the Ivory II. I am a pianist by training and I use the Ivory II and I like it's Steinway sample the most (+- 30Gig samples with 18 velocities). The included Amber piano in Sonar is pretty decent and I use it for new age sounds. Again like some other suggestion in this thread you will benefit much by changing it's EQ and reverb setting (Sonar's EQ and reverb, not the vst's setting).  The Dimpro's stock piano is about 285MB Yamaha grand sampled in only 3 or 4 velocities and it's dynamics are not realistic enough. I had also purchased the DSF volume 1 grand Piano expansion and the result wasn't any better. I have heard the DSF Yamaha and Steinway demo and felt is it is not to my expectation. To be fair, the price the only 99USD each.
 
For EP sound I used Applied Acoustic's lounge lizard4. It is based on physical modelling and it's expressiveness outclass the Sonar's included SI EP. Another plus is it uses very little CPU resources. 

joden
REALLY expensive though Konrad - for not a lot of gain over the lower priced options imo - and you need a DONGLE  !!!!

It's around $120/piano. You spent $49 for single patch, and didn't like it. Each piano in Ivory II has many variations, and it is very tweakable.
 
Regards,
Dan
 

To each his own and I won't keep plugging Ivory but $179 for a Steinway and $30-40 for the dongle doesn't seem bad considering the quality (and considering the price of the real thing).  Keep in mind that Ivory has on board EQ, reverb, the ability to choose the number of velocity layers (to manage memory) and some other effects and tweaks as Dan points out.  (It even has some synth doubling, chorusing, and other stuff I never used but that is interesting.)
 
Just this week-end I was re-doing piano and orchestration for a vocal cut in another studio and Ivory added incredible expressiveness compared to whatever was used originally. 
 
The July Sound on Sound reviews Ivory American D (American Steinway--I have Ivory II Grands with the Gemrna Steinway, Bosendorfer, and Yamaha C7).  In the review, there is a sidebar that rounds up all the comparable products from other manufacturers and it is a pretty nice summary of the top contenders.
 
I promise this is my last post on this thread!  (Now, somebody needs to bring up acoustic guitar plug-ins so I can get on my RealGuitar soapbox.)