I'd rank them as follows;
1. De Bruyne
2. Coutinho
3. Ozil
4. Mata
5. Hazard
Justifying my ranking:
De Bruyne above Coutinho because his impact on the City squad is huge, they haven't won in five games, since he got injured. Coutinho just isn't as influential to me and he's part of a worse squad (so he has the potential to make himself look even better.
Ozil above Mata because he gets more game time and has his consistency and form down over Mata who, barely gets a sniff. However, whenever Mata has played he's usually done something important and influential, which Hazard doesn't/hasn't so much. Hazard is so inconsistent, despite showing signs of previous form this season, that for now he has to reside bottom for me.
End of season prediction:
By the end of the season I think the rankings will clearly change. I'm also going to take in to account things like expected game time (which is why Mata will be bottom, because I doubt Mourinho will give him many chances or a run of games).
1. De Bruyme
2. Coutinho
3. Ozil
4. Hazard
5. Mata