There are 2 obvious choices, Ed Reed and Troy Polamalu. I think you'd be hard pressed to find anyone that is on their level when both are on the field and healthy. LaRon Landry of the Redskins is definitely up near the top as well.
Reed can take an interception to the house anytime he gets his hands on it. He's been rock solid ever since he came into the league. His ability to play the run, pass, and on special teams beyond impressive.
Polamalu is the end all be all free lancing, do it all Safety. His ability to have such a feel for the game and anticipate the play is unlike anything the NFL has seen at his position.
The last 2 years or so neither player has been healthy for a full season and at their best, but still now it would be hard to choose one over the other if you had the chance. Having said that though, who's your choice for the top one?