Question : What are the best sound libraries / VSTi to start programming ?

Try to start with a given budget, then define your needs (orchestral, voices, electronics, percs, hybrids and so on)
Then I will have a long list for you....

I agree with Eric Marquis. It's a tough question, but if you want to write orchestral material, i would suggest starting with big ensembles first such as Spitfire's Albion One:  www.spitfireaudio.com/shop/instruments/orchestra/albion-one/ 

For more modern scores that are more synth based, i would highly suggest using Omnisphere:
 https://www.spectrasonics.net/products/omnisphere/ 

Hope this helps,
Medhat

AIR STRUCTURE RTAS / VST PLUGIN
i make this with it :
the orphan