One of the problems of today's marketplace I'm afraid... every library mentioned, and a bunch that weren't, are aimed at different applications. And they are all bloody expensive! So how do you choose?
Right now I use Concert Strings 2 and Spotlight Strings, mostly because I like the user interface/work flow approach.
I also use Orchestral Essentials, love the sound, still struggling with the ensemble approach.
And I use the content included with Komplete 7, along with a couple other string libraries I've picked up along the way.
I am about to jump, feet first into Vienna - the silent stage approach makes sense to me, but dang, I LOVE the sound of the Spitfire and Cinesamples and 8Dio libraries too.
Ultimately, if you are writing to a variety of applications, and making money doing it, I guess you need more than a couple<G>...