He's not the best but he is better than Mertesacker who is a lot taller.
Kos is only just over six foot so against strikers like Ibrahimovic and Carroll, who are taller and far more aggressive, he is going to struggle, but his positional sense and awareness compensate for this.
I think with Mustafa playing alongside means we have a CB pairing that is one of the best in the PL.
This is something we have lacked for a long time and we actually have the players to have a more solid and organised defence.
We have a top quality GK, great attacking FB's, two top class CB's and a great DM, as wells good cover in all positions.
It's a shame we have waited so long, but Wenger seems to have sorted out the weak links in the defence and has filled the gaps with some very good players.