Ota is ok, but he's nowhere near Bonucci's level. That isnt to say, however, that Bonucci would look a world beater in England like he does in Europe. It's the extra pace of this league, its physicality and constantly shit reffing going in favour of the attacking side that makes certain defensive qualities more essential to succeed here than on the continent.
First, for a centre back, i think you need to be a big fucker. Minimum 6ft 2 and properly strong so tossers like Costa and Lukaku can't bully you off the ball.
Second, good reading of the game, anticipation, to recognise when it's unlikely a ref will notice a slight nudge in the back or little shirt pull.
Third, pace. Or faultless positioning if they're not hugely quick. Ideally both though.
This is why Kompany is the best centre back in Europe, he has all the above and a first class attitude.
The only central defenders who would really be good enough for us are van Dijk, Pique, Ramos, Boateng or Cahill. Apart from van Dijk who will be about £70m (and with good reason), the others are unobtainable. So we either spend on van Dijk or Pep coaches Stones who has the potential to get this good, especially with Komps next to him.