My top three would be the Moog Voyager, the Alesis Andromeda, and Dave Smith's Poly Evolver keyboard.
Everything else is a pretty dim second place.
For true analog mono, with programmability, flexibility, and bulletproof construction, the Voyager stomps all comers.
For analog polysynth flexibility, with attendant good tone, the Andromeda knows few peers.
For a hybrid analog/digital combo, highlighting the best features of both adigital and analog, with a well-designed signal path, sterling pedigree and built-in True Evil™, the Evolver devours pretenders.
All have external inputs for processing your wimpier synths or feeding the synth back into itself. (The Evolver has this hardwired, so no patch cables necessary.)
All three are truly excellent synths.
I own a Voyager. (I've owned two. I sold one of 'em at a profit, and could sell the second at a profit anytime, so it's not just the "your-investment-talking" syndrome.) I love it. For an expressive, reactive solo voice, it's without parallel. Superb in design, execution and craftsmanship. In case of a fire, it's the first thing I'd check for, after my wife.
-Hoax