The best sounding acoustic piano on an electronic that I have heard so far is the newest Performance Controller from Kurzweil - the PC3X. Right now, there are only about 70 of these in the US, general release should be in the next couple of weeks.
I have been using a Kurzweil PC2X for the past couple of years, the Triple Strike Piano variations on it were my previous favorite.
I received one of the very first of the new PC3X Friday. Compared to the PC2X, it just sounds more detailed. The sustain on the upper range is considerably extended (when sustain pedal is used), more like an acoustic grand. The biggest difference though was when I push it a bit. The older Kurzweil would play all the way from pp to a strong f, but just kind of ran out of range. The new one just goes to a stronger ff or even fff. The dynamic range has been expanded.
This is the first totally new high end instrument from Kurzweil for about 12 years. I haven't even had time to listen to all the sounds yet. Now, there is still a full Orchestral ROM PLUS a new Strings ROM - so many different variations in sound. Also a new virtual Analog synth built right into the new VAST operating system.
The new keyboard (new design - no lead used to meet EU regulations) has a different feel also. It still feels like a moderately strong piano action, but the "klunk" from the key weights is much quieter. Still seems to have a good action for playing.
Also - the basses - first instrument since my very old K2000VP that really has a solid old-school bass that blends in the mix but has that deep ability to walk empty cups right off tables.
I've only sold ONE of the Kurzweils that I have ever bought - I had a chance to get a PC2X at a very attractive price used, bought it, and sold my SP88X stage piano. This instrument is used in my home living room for all three players in the family.
At church, I have a 15 year old K2000VP and a PC2 (76 key). My shop/studio has the new PC3X and a K2661. I haven't found anything yet musically that I can't do with at least one of them.
If money was absolutely no object, the Bosendorfer CEFS electronic will probably be the finest sounding electronic ever built. It uses the complete action including hammers of a Bosendorfer Imperial Grand; into a very complex computer system. In final test now, expected release very soon. Price not yet released - estimated to be above 50,000 euro. Defined purpose is to produce a world class instrument that is portable for concert artists to use for both practice and concerts.
Jim