Sorry, if you are rating the best Dolphins of all time, there's a bunch of guys ahead of Zach. Don't get me wrong; I loved the way he played the game but here's a list that you could make a good argument of players ahead of Zach;
It's hard to compare Csonka to Zach, or Griese to him, so I'll just go with the defensive guys you named.
Buoniconti - Great player, obviously. In the HOF. However, he played for the Patriots and had the best supporting cast possible in Miami.
Scott - Great safety. Most under rated Dolphin of all time. That's a tough call. He came through in the clutch as well.
Madison - If you are going to put Madison on this list, you mine as well put Surtain too. In any case, I don't think he played in Miami long enough to compare him to ZT.
JT - He's a Jet.
To me, I think the top contenders are Buoniconti (although, again, he played in NE for a lot of years), Scott and Manny Fernandez. Maybe it's impulse because he just retired, but I think Thomas has to top the list of all time Dolphin defenders. Just me though.