Wlad, I think has a rather substantial claim. Both should, but for the sake of debate, Wlad ought to get a slight bump over Ward.
The man reigned over a division and fought everybody. It's not his fault people criticize his reign, and if people criticize his style, then why do other fighters get such accolades? They according to their mentality and physical build. Fair play.
It's worth mentioning that there have only been a couple of really good heavyweight eras: the 60's-70's and the 90's. Just because a heavyweight didn't compete in those times, doesn't mean he should be written off so easily.
Wlad belongs. He had staying power and dominated. He came back from big losses that would have quelled lesser men. He fought the toughest guys he could at their absolute peaks. Rocky Marciano retired rather than fight Patterson, but Wlad went right after Tyson Fury, and then - after an extended leave at an even older age - took on Anthony Joshua in one hell of a swansong.
Ward beat some legit names, but he was inconsistent. Froch, Kovalev, Dawson, Kessler, Abraham. Those are sturdy wins. I respect those for sure, but there are other criteria beyond quality of opponents (even though that is a really big deal).
Like I said, close, but I believe Wlad edges it slightly.