I'm sorry but you undermine all your credibility in saying that Ali had weak opponents. Sonny Liston may have been past his prime, but Ali fought legend after legend in the undisputed greatest era for boxing. Any one of those guys, Foreman, Frazier, Louis, Norton, and Liston even at that age, could have or were very deserving heavyweight champions in any other age.
Tyson had the look of the greatest ever in his early years, but it is hard to say exactly because his early competition was weak and later on with his problems he was never as well trained or mentally strong when he faced good opponents like Holyfield, Douglas, and Lewis. He was almost certainly much stronger than the pre-scientific (and probably pre-steroid) lifting boxers. And he was fast and had a killer instinct. Ali never had anything like that knockout punch.
The Klitschkos must be pretty good to dominate this century the way they have, even without fighting each other and with periodic retirements, but they have steamrolled their opposition and it is just hard to say. An early doping violation in kickboxing(?) also tarnishes their reputation.