Depends on your style of music...
I like Reasons Subtractor alot, although the sound is kinda plasticy.
Pro53, and FM7 are my most used ones as far as stand alones.. Within Cubase I like Jungalist
ALOT
download some demo's @
KVR
click on instruments when you get there, and search for what your looking for.
I havent been able to find any solid Orchestra or Timbre type soft synths- most are just digital noise that your gonna have to sculpt with.
Fm7 and Pro53 is by far the best for my needs- I use them with my UC-16 and Oxygen 8 for knob wrestling.