We've got a bunch of favorites. I personally have an Ensoniq Mirage which I love for it's gritty character, a Korg DSS-1 which actually has an amazingly refined and mellow synthetic character, and an E-mu Emax II which has some cool synthesis abilities and some really excellent digital filters.
I'll also be adding an SP-1200 to my arsenal soon, I'm really looking forward to that.
Those S612s are hard to find! Good luck with that.