*Rant Mode ON (take it with a grain of salt)*
It really ticks me off when people make comments like "I want a real instrument sound off of a synthesizer"... Ummm, hello! A synth IS a real instrument. They are capable of many timbres and textures, evolving sounds... sounds that can't be obtained by any other means other than electronically. Take them for what they are. I don't understand why people whine that synths don't sound like real acoustic Violins (for example)... this makes no sense. I mean you wouldn't expect a Violin to sound like a French Horn would you? A synthesizer sounds like a synthesizer. Different models of synths sound different, but they are still synthesizers, i.e. REAL instruments in their own right, and they do what they are designed to do very well, acoustic instrument emulation is not really what they should be used for. They are not pianos either (even though they have a keyboard... an accordion also has a keyboard, but doesn't sound like a harpsichord, or a piano for that matter).
*Rant mode OFF (I think you got the idea)*
Now... if you want the sounds of real acoustic instruments, nothing will come close to actually playing/recording those instruments. You can emulate the sounds of these instruments with samplers and synths, but those will always be emulations. Also to aid the realism, you have to pay attention to the performance. For example, you can't pick a violin sample, and play close spaced 8 note chords and expect it to sound like a violin, no matter how good the sample is. Having said that, a well recorded and programmed Gigastudio library would probably fit the bill. For pianos, I've always liked the Kurzweil pianos, especially their triple strike versions, but tastes vary. I've liked the playability of the stock Saxophone patches on the Korg Triton, especially when you use the ribbon to modulate filter cutoff and growl. Overall I've also been happy with the Orchestral ROM for the Kurzwel K2x00 series workstations. But again it will do you no good to have great samples if you can't imitate the instrument's performace characteristics. There is a lot more to emulating acoustic instruments than just the timbre.