These are the ones I have (in some form or another)
CableGuys Curve (CM Version)
Aalto (CM Version)
Absynth 5
FM8
Cakewalk Z3ta+2
Alchemy (Free Version)
Reaktor 5
Zebra 2 (CM version)
Massive
Synthmaster (Free Version)
Traingle II is actually a hidden gem (by rgc audio, or Cakewalk?)
I don’t even think you can download anymore. But I used it several months back to create some interesting sounds. I’m really thankful I grabbed it when it was still around. Really diverse and capable synth 
I would look for it anywhere you can.
I think all of the above are great for sound design. They just have different uses and strengths.
Without a doubt, Reaktor 5 may take the cake --but that may not be fair. It’s really an environment where you can create synths (kind of like Max or Flowstone). It can’t really be compared to the other synths. But none the less, it is brilliant for sound design.