I have just started looking at this thread. I would love some modular hardware and have built many a fantasy system from
http://www.synthesizers.com/ and others but I don't have the space even for a modest system (and I don't have the funds for an extensive system.)
VSTi to the rescue. I have
XILS 4 which takes me back to my first encounters with synthesizers, I had access to a EMS synthi A suitcase model back in the 70's and it was there I learned to understand and love analogue synthesis. How authentic XILS 4 sounds I can't say after all these years but it is more stable and does not exhibit the tuning issues that made the original more of an effects synth than a keyboard synth. Also as it emulates two EMS synths linked together it is sonically far more versatile.
My favourite at the moment is
Softubes Modular VSTi. This is a virtual Eurorack made in conjunction with Doepher with an expanding range of modules from Doepher, Inteligel and Rolli along with a range of Softube utility modules. The great thing is you can have as many or as few modules loaded as you want, no need to clutter the GIU with unused modules and if you want 8 VCOs, no problem.
To my shame I have never got my head around Reaktor, I keep on meaning to try to create some ensembles but it never happens.