I began using synthesizers in 1982. I started with the Yamaha CS-70M analog synthesizer and then moved on to FM synthesis; I am one of the first American composers to score an animated TV series (The Adventures of Gumby) with only synths in the late 1980s.
I've used many software synthesizers, and still do, including FM8, Z3TA, Tera, Massive and Dune. Dune 3 is the most sophisticated and interesting softsynth I've used to date. The sound quality and the range of sounds that can be produced is incredible. The digital sounds have a clarity, beauty and purity to them that is unmatched. It is not only one of the deepest synths I've used, in terms of number of programmable parameters including occillators, waveforms, LFO, envelopes, arpeggiation, signal processing and many more, but is also, unlike many synths, not difficult to edit or program from scratch. It integrates seamlessly into the DAW I use (Sonar/Cakewalk) and I've never had it crash on me. The ergonomics are logical, the signal path is not difficult to learn and understand and the synth's visual appearance adds to the synth's programmability.
If you're looking for an incredibly versatile synthesizer with a lot of wonderful factory patches, fairly easy to program and with many musically useful timbres, I highly recommend Dune 3.