It sometimes depends whether ur listening or playing .... for
listening, the rich, resounding notes from a Stradivarius violin
really makes it for me ...
for playing, the electronic piano or Yamaha's Clavinova series is
the one ... u get the feel and timbre very close to a piano as well
a vast resource of other fantastic voices ... can synthesize,
sequence, play guitar, bass, harp, organ, FX sounds etc. on it as
well ... a complete musical instrument on its own. 
