Difficult choice...of course one would like them all:-)
I think its the Matrix12/Expander which makes my mouth water the most.
I'm surprising myself as I write this, but I don't see much point in doing a Chroma (I have one downstairs in my cellar...dead, unfortunately.. and I've loads of sys-ex files for it...I got mine MIDIfied). Sure, it sounds beautiful, but bearing in mind that its control panel is awful (just 52 touch switches, one slider and a 2-digit display), and the fact that all supposedly "continuous" controllers are 8-bit resolution (e.g. the shortest envelope time, other than zero, is about 20ms...nowhere near fine enough!)..for me there's not much point in doing a "faithful" emulation of it... seems to me more like a job for a Reaktor ensemble.
An EMS Synthi A would be great fun ( OMG, I actually *had* one for about 3 months...erm..30 years ago, yikes!!).. but it would definately need an Arturia-style "under-the-hood" type access to a button marked "keep me reasonably in tune, please!", or at least a variable parameter to select the length of time before the tuning rendered it unplayable!:-))
Yep, more I think about it, Matrix12/Expander it is!:-)