Crystal... 3 oscillators, great modulation, a whole bunch of built in delays, does everything from analog-sounding to sampled to FM to grain, plus wavestation style wave sequences, automatic tempo-locking...
There are a lot of great soft synths not on your list. I checked NI becuase I like a lot of what they do but some of my other favorites include Tassman, Spectrasonics Triology and Atmosphere (verry good synths), VirSyn stuff like Tera and Cube, the rgc stuff like Pentagon and z3ta+, LinPlugs Albino etc. Of course, the ones you listed are good also, I have some Steinberg synths and Reason. I doubt if I could limit it to one but I am very fond of Atmosphere, Albino and Absynth. I think I'm really going to like Cube as well.
I am not considering emulations like Lounge Lizard or B4. Similarly, I am not considering samplers like Giga or Kontakt which really have a different purpose.
I think you make a good point here. As a result, I would change my favorite from B4 to FM7. I have found the FM7 to be usable in almost any mix. It has a wide range of capability.
I own Tassman, but have not really explored it much. Since I am in Education, I received the Educator's discount and saved a lot of money on it. I really should take some time to learn how to use it.
Another program that I am finding extremely useful that does not fall in the synth category is Groove Agent. I think this program is incredibly well written.
I think the FM7 does a great job with FM synthesis. Tassman is interesting because it is physically modeled, so there are some fun models included. I don't program it a lot, but the sounds that come with it are pretty musical. For programming, I really like Albino... it has many interesting programming features (like changing the pulse width of any of the analog waves). As stated in my post... Atmosphere is a great synth but is not the programming beast that some (like Tassman) are. I like'em all!
I usually don't find myself using softsynths that often. But when I do, the one that I always turn to first is Absynth. I love how easy it is to get evloving atmospherics out of it. Admittedly I also like the fact that version 1 aliases... haven't upgraded to 2.0 because of this.
I'm not a keyboard player,but I just got Kompakt and I'm very impressed.I have gigastudio and various free and included with Sonar softsynths.Kompakt has replaced them all.