East/West has a lot of other great libraries. Gypsy, from what I've heard, sounds really good, so I agree that it's definitely a library to consider. Other great East/West VSTs include Pianos, Ra, Stormdrum 2, Silk and Symphonic Choirs. Others (Goliath, Fab Four, Voices of Passion, Ministry of Rock , etc) I've heard bad stuff about, especially Voices of Passion. I haven't heard anything about The Dark Side or Spaces Reverb. Oh, and the Hollywood Strings/Brass/Woodwinds are glorious in both sound... and price.
East/West aside, Omnisphere is a brilliant VST. I consider it the best plugin I've ever worked with. It has 8000 sounds, ranging from guitars to choirs to mallets to pads to lead synths to arps to everything in between (except a cowbell - yes I've checked), and some excellent synthesis features. It's so deep I've barely scratched the surface and I've had it for something like half a year now, and it's child's play to integrate it in orchestral songs, or use it for basically any other genre. However, at $500, it doesn't come cheap. Other VSTs from Spectrasonics like Trillian are great too, and cheaper.
If you want something from Native Instruments, I'd highly suggest getting Komplete 9. From the looks of it, it's simply not worth getting a Native Instruments plugin if it's not from Komplete 9. ONE Native Instruments plugin can cost hundreds of dollars, especially if it's a big one like Kontakt. Then, to get THIRTY-THREE of their instruments, it costs 650 dollars. Basically if you buy two of their best products separately then you've already reached more than it would cost to buy those two and thirty-one other products. Komplete 9 comes with loads of fantastic plugins such as Massive, Absynth, Guitar Rig and Kontakt.
Then there are some great free ones that Elitistinen did a good job of pointing out. Let me add that these guitar/drum samples are just excellent (listen to the audio demos!) and they're entirely free if you have Kontakt. Monster Staccatos is also an awesome free library with some wonderfully meaty string staccatos, but again, you can only get those if you have Kontakt.
If you're into soundfonts, this is a great place to start, although they probably won't be that useful for you if you have East/West. If you're on a strict budget, Magnus Choir is decent and much cheaper than the big names like Symphonic Choirs and VOXOS, but obviously not nearly as good.
Finally, I'd like to give a shout-out to the mda Piano or mda ePiano, both pretty darn good free VSTs if mixed tastefully (especially the latter), and the MiniDiZi and MiniErHu, two beautiful-sounding chinese instruments that are entirely free. I don't think you can install these on a Mac though because as usual Apple like being awkward and different from everyone else and have this stupid Audio Units thing which makes it impossible to download VSTs that don't need an installation (i.e. just dll files), and being small and light, the last four I've mentioned just come as individual dll files as far as I know.
I hope I've helped!