Old age and lack of health. City haven't had their best players healthy/not suspended at the same time. Gündogan, Aguero, and Kompany have missed extended time. Fernandinho and de Bruyne have also missed a few matches. Pretty sure none of Chelsea's best players have missed (m)any matches. That's partly a function of not having European matches to tire them out and make them more prone to injuries.
I would definitely rate Chelsea's squad over Arsenal's. Better goaltender, far better midfield, better attack. Their backline is maybe a bit worse on paper but they still manage to concede less goals. Probably because Wenger is a mess.
I don't think so...
Costa = Sanchez
I think this has been a wash, Costa leads the EPL in goals and has rebounded nicely from the disaster he was last season. But Sanchez has been a revelation as well and definitely should be the leader in POTY award currently. I will give the slight edge to Costa since he has the history of being a great striker, but Sanchez has closed the gap IMO.
Hazard > Walcott
Not even close IMO, but with that said, Walcott has had a very solid season so far. Perfect time to sell him as his value is as high as it can be...until he gets injured again.
Willian < Iwobi
Chelsea fans were saying about how useless Willian was last year even if he had a good work rate. Willian has Iwobi beat in terms of play at both ends of the pitch, but if we're talking about the true strengths you need as a winger, Iwobi already has him beat and will only continue to improve. This isn't to say Willian is a bad player, but just doesn't have creativity of dribbling skills to be a winger for a club like Chelsea IMO.
Pedro =< Campbell or Welbeck
Of these three players, give me Campbell over any of them, but sadly Wenger thinks he's trash and sends him out on loan every year. If we compare this year's Pedro to last year's Campbell, Joel still has him beat, only reason why people think Pedro is the better one is because Campbell get literally zero opportunity to show himself.
Fabregas < Oezil
I highly dislike Oezil's lack of desire at times, but he is clearly the better and more talented player. This doesn't mean Fabregas sucks or anything like that, but Oezil is stepping into his prime while Fabregas is stepping out of his.
Kante > Coquelin
By a pretty good margin, but again, I think Coquelin is underrated on this board. He is good at what he does and that is being a mobile CDM with good tackling ability. You all will probably disagree with me, but Arsenal can win the EPL title with him getting a good amount of minutes.
Moses <Cazorla
This might sound weird, but Moses is actually the better player since he actually plays. However, if each player was healthy, Cazorla is easily the better player. He has limitations due to his small stature, but his creativity, playmaking, and work rate would run circles around Victor.
Cahill > Mustafi
Honestly, the only reason here is because Cahill has EPL experience, but Mustafi will definitely overtake him soon. Cahill has been on a downward trajectory and currently is overachieving IMO.
Luiz < Koscielny
It really depends on how much you value Laurent. Personally I think he is another player who is severely underrated on this board, but each of these players have their own demons IMO. Luiz is better going forward, while Kos is better in terms of pure defending.
Azpilicueta = Bellerin
Pretty much another wash, Azpilicueta is a better defender while Bellerin is better at going forward with the ball.
Again this Chelsea team is clearly overachieving, but that isn't meant to be a negative. I would rather be in 1st and overachieving than to be in 3rd, 4th, or 5th. But with that said, how long can they preform this well for the season? This might be their only really chance to win since they will definitely be in CL competition next season. They had some solid wins against Man City and Tottenham, but have played pretty much crap besides them. And even the game against the Spurs, I believe that was with a Kane-less squad. No arguments from me when they wrecked Man City, that was a solid win. In the case of Arsenal, you can once again say that they do have the better team, but injuries every single year prevent them from truly making an impact. That and you have a ****** as the manager. Take for example Oxlade-Chamberlain, he's better than any single winger on Chelsea's bench, but has been in and out of form and just got injured since Wenger has zero ideas in how to use his players properly.
Kompany is the best player on all three teams? Is this 2012? Even when healthy, Kompany hasn't been a top level CB for some time now, his spat of injuries destroyed them. He's far from being the best overall player (Hazard, Aguero, KdB, Alexis, Ozil, and so many others), if that's what you're saying, and far from being the best CB (David Luiz and Koscielny both above him), if that's what you mean.
Kompany really reminds me a lot of the situation which Kris Letang was in a couple years ago. When Letang was going through the injury bug, not one single team would have taken him or his contract. In a sense he was not the best player on any club in the Metro division. However, since he has gotten healthy and put the effort/time into actually playing, you can say he is the best defender than any team in the Metro. Kompany should be seen in a similar sense IMO. Is he best player of any of the three squads? No since he's been constantly injured the past couple of seasons, but would he be the best player giving him a month of injury filled playing time? Yes he would.