I know this probably doesn’t count as a straightforward, singular ‘synth’ as such, but the units available in Reason (especially Reason 4) offer an infinite range of possibilities when used either in standalone or ReWire mode. Thor obviously springs to mind, but I find even the most ‘basic’ synths like Subtractor and Maelstrom offer phenomenal scope. Taken together with the potential of ‘Combinator’, Reason can be thought of in many ways as a plug-in instrument like no other. In my experience it is also very economical on processing power too…