Sagna, Ivanovic, Richards and Walker are the best in the league and way ahead of anyone else.
Other than that it's a very average at best position ability wise and Simpson is no worse than the majority.
Ivanovic isn't a good full back. Certainly not at the lever of the other three players you listed. He's mega slow to turn, which is crucial.
Alan Hutton is a premier league right back. That says it all about the lack of quality in the position.