IMO the answer to this question is Sam Madison. I give it to him for longevity and production.
If you look at it for who played at the highest level at CB for the Dolphins, like saying the most dominant CB we've ever had, then Surtain, and possibly Vincent, would certainly be right up there with Madison IMO, for the extremely high he played for 3-4 years.....but Madison had an almost equal 3-4 year run AND had more on either end of that run, now that Pat has moved on.
Also, if you were to view the question as who had the best career who was at some point a Dolphin....then Troy Vincent is probably your winner. It's sort of like saying that Deion Sanders is the greatest all-time Raven CB though.
I'm only judging by what they did as Dolphins, and Madison gets my vote.