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.