There was a thread here last May (among many others I'm sure) about favorite synth plugins. At some point in the thread the most mentioned ones were Diva, Omnisphere, Sylenth, Alchemy, and Massive.
MusicRadar had a reader poll a little over a year ago; the top 10 were Omnisphere, Zebra, Sylenth, Massive, Reaktor, Alchemy, ACE, Nexus, ImpOSCar, Absynth.
"Most popular", "most advanced", and "best sounding" don't necessarily overlap (though many of the above do show up in the intersection). Less popular but technically and sonically interesting recent softsynths include Chromaphone (physically modelled percussion), NI Razor (additive, hundreds of partials with a macro interface), NI Prism (hundreds of resonators with a macro interface). Aalto (semi modular with some Buchla inspiration) has gotten some good mentions.
If I take the 2013 literally, the best are Diva, Zebra, ACE, and Bazille -- U-He has recently published betas of new versions of all their stuff. Bazille is a virtual modular (akin to ACE) with the twist that the oscillators are FM operators (with phase distortion and "fractal resonance" as well) that's been in beta for years that might finally see an official release this year. Until then it's free, and well worth checking out.
To me, the oddities of synthesis methods and the benefits of sample RAM/ROM are where plugins shine. Razor, Prizm, Aalto, Massive, Kontakt, Omnisphere, Trilian, String Studio etc are inspirational kit right there. For simpler methods like subtractive, I'd prefer a knobby box, (although if it sounds good to my ears I'm not an analogue snob).
Additive, granular, FM/PM, waveshaping, massive-library S&S... That's the true highlights of software synths. Big or downright weird soundscapes and sonic journeys, metallic, synthetic. Added to that the near-infinite modularity of effects chains... Lovely.
Sigh, I wouldn't count on that. Urs Heckman, Jan 15 2013:
As of now I can't imagine Z3 shaping up this year (2013) anymore. The required work on the gui framework is massive, and that's before the actual design process. I'm not too scared of the dsp work, but the overall design of features and workflow is a frightening task.
no but Aalto is a good one
also I'm quite fond of the PPG for the iPad
to add to the examples already mentioned
one thing: these are MUSICAL instruments, "best" is subjective
"sound good" is subjective
"technologically advanced" does not automatically equate into "a more capable instrument"
that's why I love music production
my 2 ¢