Native instruments do the best softsynths IMO.. they are not a very good value though unless you buy the komplete package. Although the sound clips/advertising is mostly geared towards hip hop or electronic musicians, NI products are chock full of excellent vintage synth sounds.. especially FM8 and Massive.
NI Komplete also includes Guitar Rig, which is the best and most advanced amp simulator and effects engine there is (software or hardware), so if you use it for that also its a really good deal.
Aside from NI, spectrasonics omnisphere is very good.
You can get tons of vintage/80's synth sounds for free, but the quality of them is not up to par with the commercial stuff. Most DAW software like Ableton, Cubase, Sonar, etc come with many of these type of sounds, but again the quality is generally not as good as what you get from NI or omnisphere.