Probably Vitali (even though he was past his peak and only active for four years of it) with Povetkin following.
Wlad and Vitali were a tag-team act who avenged each other losses so you'd have to go to the best guy who was Povetkin
The bookies odds say David Haye, who was given a 35% chance to Povetkin's 18%. Haye was regarded as having more speed, power and killer instinct than Povetkin, though both sh*t the bed in their own way. Povetkin had the best collection of wins during that period, better than Vitali's on paper, though Vitali's wins were more dominant and he would have been favoured to beat Povetkin or Haye, even in 2012. At some point in 2013-15 Fury eclipsed Wlad.
The question is not the best asked. The second in that era would be Vitali Klitschko. Now, on this forum, it was voted (and I voted that way) that in the fight Wladimir Klitschko Vs Vitali Klitschko would win Vitali. That is why I choose Povetkin as the second (beat Chagaev).
Vitaly is the obvious choice for 2nd best and had they not been brothers we could have seen a champion emerge as being undisputed.
2nd Best Heavyweight Of The Wladimir Klitschko Era is Wladimir Klitschko. First is Vitali Klitschko. https://www.boxingforum24.com/threads/wladimir-vs-vitali-who-wins-the-battle-of-brothers.679817/