According to some Koscielny has been our best defender....I agree with you though Vermaelen is pretty good so he's another player who's important...we've been a lot better since he's been fit.
Koscielny is average at best though IMO, a player made to look good by players like Vermaelen, just look how awful he is when Vermaelen isn't around...also cost us a goal against Man City.