PianoTeq is considered the best piano sound. some even argue it is better than a real grand piano (it never goes out of tune) - but only if you have a great keyboard (several thousand dollars!), and sound system (more $$$) They are not known for organ sounds though.

If you want organ sound, then the software to look at is: grandorgue, Hauptwerk ($$$, but reported to be great), Aeolus (simulates pipe via math, the rest work on samples of real organs). There are probably others, but the above is what I see come up most often when this is discussed.