First things first:
The secret for a good sound lies in the arrangement !
When money doesn't matter, buy them all !
I had good luck in layering a lot of sounds in GigaSampler, it depends on the project.
Sometimes you need upfront sounds, then it has to be smooth and warm or whatever.
Vienna and Sonic Implants is a good start for film music.
The new Apassionnata from VSL are awesome.
Siedlaczek and Miroslav sound a lil bit too dated meanwhile, but Miroslav has some very nice patches for layering, eg Violas and Celli, mmhh.
Siedlaczek has some very good string detachés
The detachés in the Garritan are also very nice, but the rest is horrible. The "deadest sounding library" imo. Sounds like bagpipes.
Long strings in EWQL are horrible, but the short ones can be very good. Horns in EWQL are fantastic, as are the Percussions. But EWQL always sound "roomy", like it or not.
Vienna Horns are very good too, Woodwinds in Vienna Performances are so good, that even a flute player asked me, who had played.
and and and
Reverbs are very important ! Somehow, Softverbs never make it for me...
All in all, I have to say, that it takes quite some time to learn a library. The better you know it, teh better the results. Like an instrument
I always and up with a big crossover of everything.