Nobody has mentioned the Minimoog Model D and its modern variants(Voyager, Sub37, etc.), so I will. As synthesizer instruments go, it's astoundingly hard to match the number of "sweet spots" the genuine article has. There are lots of synths with more complex architectures, polyphony and timbral possibilities, that are more affordable and ship with lots of presets, and there are lots of bad digital knockoffs that don't reproduce the sound correctly or correctly handle parameter changes, but an original one in good condition sounds good from nearly any starting point and allows for continuous development of a sound just by holding one note and turning knobs. You don't "program" a Moog and then play, you "perform" it as you play - and that's a key difference between their style and what most other synth makers put out.
