But with Pep coming in there should have been a response and at least a challenge for the title.

There was a challenge for the title but chances of Chelsea dropping enough points for others to catch up is not realistic at this point.

You still haven't answered my question: Which one of Manchester City defenders would you have at current Tottenham or Chelsea teams? Which one of Manchester City fullbacks would you rather see at Chelsea-Spurs-Arsenal-Manchester United-Liverpool right now?

Mangala left because he's not good to play Pep's system where players are involved in the build up and are active participants in the game while they also have to defend. They hoped Kompany to recover and tried to buy Bonucci but neither happened and Pep now plays with Kolarov occasionally filling in as a centreback. Still, while Stones and Otamendi have been improving, you still need other players.

Mind you Chelsea and Liverpool (and now Spurs also) play one game a week (except for the FA cup schedule, which City are also a part of) and have an entire week to prepare for the games while City and United haven't had that luxury so far.