A lot of realy cool synthesizers mentioned!
Well I have a tendency towards lazy and cheap, so my favorites are two that I've owned for decades, an ARP 2600 and a Korg MS-20. Both have been reissued, but I've not played with the newer versions. I have heard good things about them.
In terms of something just a little more modern (as in still in production) I picked up two new synths in the last couple years...
The Roland SE-02 sounds wonderful. I am still struggling with programming it, the "user interface" could be a little clearer<G>. But it sounds beautiful! No regrets!
The Pittsburgh Modular Microvolt 3900 is a bit of a departure for me, and that was why I bought it. I had set out to build a modular synth, but something about this caught my ears. It is described as a "West Coast" style synthesizer, and it is a new world for me. Oddly enough, I find it a little easier to program than the SE-02. A little anyway.
Familiarity means that I still find the ARP 2600 and the Korg MS-20 easier to work with. Not sure I'd want to take either of them out of the studio though. The other two are ready for the road!