I think Otamendi has been pretty good this season all things considered. He struggled for a bit when he arrived and people seem to be a little slow to adjust those initial impressions of him. He did a carrying job for City's defence while Kompany was out though. I'd expect him to be back to his old level next season.
Mangala is one of the worst players I have seen though. He's so fucking shit it drives me mad. Someone needs to investigate what happened to the money from that transfer. I bet there was some shady stuff going on there.