Although high on the learning curve (unless you want to use just the presets), I think Native Instruments Absynth deserves a high rating in the sound quality, stability, and uniqueness category. No other synth can produce, IMO, the variety of the sonic landscape that Absynth can… from classic synths emulations to "what the fuck is that sound" I heard a four song demo with just Absynth and Battery and it was pretty hard to believe at the time… however, I do have to mention that the synth bass lines were EQed via a real Outboard Pultec, and an outboard 1960s chamber reverb was used on many of the Lead synths parts and lastly all tracks were fed into separate Neve channel strips (that I could not identify, but were turned into rack mount gear…crazy).
In the "free" category… Crystal is a pretty cool tool to have around, even if it's just for transitional effects from one song section to another.
