I think when it comes to hardware and authentic sounding acoustic instruments my Yamaha Tyros 5 is as good as it gets... However, the Acoustic piano sounds are underperforming. I like most Tyros 5 SA2 accoustic sounds better then Rolands naturall acoustic sounds, which also are quite good...

Korg has some nice guitars and piano sounds, but very weak on brass wood and other accoustics
Ketron really misses the nuances that Yamaha allows to create authentic sounds, but still as plain sampled instruments they sound great
Kurzweil never got much deeper with their VAST architecture
Nord, doesnt do much accoustic sounds except for pianos
Etc etc etc...

If you want really authentic sounding acoustic instruments, then you are better of using Software VSTs.. Just try the string ensembles in LASS(kontakt library), or the pianos in Pianoteq and the many other virtuall instruments and you realise that some virtuall instruments currently sound better and more authentic then some of the less stellar cheaper orriginalls.