galaxy is good!!! i was just working on a song the i used galaxy pianos and ik multimedia Philharmonik

 

Imperfect Samples Braunschweig Upright is by far the best sounding piano.
It's a big library but you can use the patch with fewer layers and save the bigger ones on an external drive. and use them when mixdown or something.
Anyway. this is the library to go with. Or the white baby grand by the same company. Personally i prefer the upright.

 

Baffled me some of you even mention pianoteq. That one sounds so bad.
Anyway, best way to create realistic orchestra is to use layers of different instruments. The best ones that sound good right away I think are Symphobia and LA scoring strings. VSL and East West are nice to have for a more complete sound and to be able to use solo sections. The more choice you have, the better you can adjust to every piece. If you always just use one library, it's not gonna have everything you need.
For the piano's I always use either Galaxy S or Emotional Piano, both of which are stunning. Ivory is nice as well, but only if you want an extremely clean and perfect piano sound without extra noises and stuff that makes it realistic.