The options are overwhelming these days, and many of such high quality. For orchestral, I still like the old Vitous libraries for the warmth, realism, and natural recordings. The VSL libraries are recommended by many for the playability and programming of articulations, with a completely dry (some might say "sterile") recording style in contrast to the hall recordings of the Vitous.
For piano, the Quantum Leap pianos are superb, as is Ivory (I prefer the sound of the former, but you should listen to the comparative demos online and choose according to taste and style of music). It's also worth checking out PianoTeq.
For other sounds, I would recommend anything on the Play libraries by EastWest (soundsonline.com), as well as the Spectrasonics libraries (Omnisphere, Trillian, etc).
If you are looking for excellent keyboard (organ) sounds, check out Scarbee's products, which are soon to be distributed via Native Instruments exclusively. NI already took on his top-notch electric bass libraries (highest recommendation for those).
Kontakt is certainly an excellent starting point. Kontakt has become the de facto standard format for sample libraries now, and with good reason. It's a very deep programmable (scriptable) engine, with excellent sound quality. The included library is very impressive, and is probably the best single starting point if you are interested in expanding your sounds.
mk3