I'd probably say Vidic because from a subjective point of view, he's my type of defender - all action, leader, organiser, awesome in the air and with his tackling. He was a magnificient signing by SAF and it's great to see him as our captain. He still has those attributes but to a lesser degree, sadly, due to injuries. But with him in the side from 2006-2009, you never really worried.
Ferdinand though, is one of those defender who as said above makes the job look easy, probably doesn't get the kudos he deserves. But last season, we wouldn't have won the title had it not been for his skills. Skills which as he gets older, he can maintain as his games doesn't involve the intensity that Vidic/Campbell need.
Campbell was immense also but Vidic for me just is a name who when he was, or is on the teamsheet, brings calm to my nerves.