I'm no soft synth expert but the one I have had access to that I liked the best is absynth
I'm also biased against sample packs - they are available to everyone and therefore automatically lack originality. Just my opinion.
Didn't see anyone mention this, but I always liked Absynth for making ambient sounds. If you really get into the nuts and bolts of the routing/sound creation, you can get some really cool sounding stuff out of it; it's a great starting point.