On a side note, I'd like to see the Klits get their just due. I don't understand the distain for them but like em or hate em they are dominant heavies and both belong in the all time class IMHO.
I think both Klitschko's would have been a handful for any of the all time heavy greats.
Forget the older timers, as most were basically cruisers, and the size difference would have been just too much to overcome. Who could've challenged them from the black and white era? Marciano was 190lbs soaking wet, so come on. Joe Louis 210 or so...Louis had some fluid skill, so maybe he could've done some things against them. Schmeling 190lbs? Jack Dempsey, ugh...
As for the more recent 'greats'. Joe Frazier couldn't have matched-up. Foreman in his hey-day could've matched them strength-wise, but I think the Klit's are more skilled and would have gotten inside those wide shots. Ali I think could have found a way to beat them.
Larry Holmes would have been tough, again because of skill level. Jab and composure. Even a prime Tyson never fought anyone like them on his way up. Tyson was great no doubt, but he was also the beneficiary of a very deficient heavyweight division (Pinklon Thomas, Tony Tubbs, Trevor Berbick, etc.)
Lennox Lewis proved that another big strong skilled heavy, like the Klits themselves, seems to be the only prototype that can challenge them.
So, yes I think both certainly need to be included in any discussion about all time heavy greats. They are the two-headed dragon of today's heavyweight era...and they've pretty much cleaned out the division. It seems the only worthy heavyweight fight today is for them to fight each other. That will never happen, nor should it.