Welllll...lessee...
Memorymoog+. Absolutely. Given how breakable the original hardware ones were, with the Autotune failure issue and myriad other problems, this would provide a RELIABLE version that never exactly existed.
Korg PS3300. The poly-monster patchable. Rare as hell, and therefore just about unobtainable.
Something based on the Bell Labs/Alles engine. Crumar GDS, Synergy, etc.
This might sound silly, but a CZ-series Casio, provided that the awful programming interface was majorly assisted by loads of 'under the hood' abilities, extra (or user-definable) waveforms, etc. Even crazier would be an emulation of the original 'CZ', the prototype Cosmo that was developed in conjunction with Dr. Isao Tomita.
ARP 2500. I'll toss my vote in for this one as well.
Same goes for a Buchla...in this case, a 200-series Electric Music Box (large-scale!) circa late 1970s, maybe with one or two replications of some of the 200e modules tossed in.
...and lastly, one for the steampunks: an emulation of the original Cahill Telharmonium. Yeah, let's wind the clock ALL the way back! Given that all of the diagrams for this were listed in Thaddeus Cahill's patents for the instrument back around the turn of the last century, modeling it should be relatively easy as long as the weird behavior of the tone-mixing coils and dynamo-based oscillators could be factored in. Yes, I know that this _presumably_ begat the Hammond organ; I would argue, though, that given the voltage levels, various nonlinearities and so forth (which would have to be coded for), this thing probably sounded not a whole helluva lot like a Hammond. Plus, the controller would have to be contended with, as the original had quarter-tone capabilities which may or may not be doable in a typical MIDI environment without a special controller. We have the emulation of the 'end-all' synth (Synclavier), so why not the 'be-all'?