Of the many instruments produced by Sequential Circuits, the Prophet 600 has always sounded to me the most like a DSI synthesizer. I really like its character. Interestingly, I've heard many P600 demos that sounded so much like a Prophet '08. I've never associated the Prophet 5 sound with the Prophet '08, but the Prophet 600 is another story. One instrument could easily substitute for the other, due to their sonic similarities.
The P600 has a fairly basic feature set, but the good effect this has is that its users tend to make music with it, rather than endlessly tweak its parameters and make elaborate sound effects. That's just the sort of synthesizer that appeals to me.
Here are some examples of the character I mean: