Just wanted to drop my 2 cents for a different opinion. First of all RIP Champ. Its one of those ironic situations where the this beautiful gifted, loquacious, athlete gets trapped in a body that doesn't cooperate. I met him a few times. You knew you were in the presence of greatness. His slow movements and quiet speech only made you pay greater attention. I am not an Ali fan. I think the rope a dope is the worst move ever in boxing. Yes, it worked against foreman, but it worked so well Ali continued to use it for the rest of his fights. If you recall, he would come in to a fight out of shape and just let people whale away at him until they got tired. Horrible, horrible fights. Frazier was his friend, gave him money when he was down, stood up for him to get re-instated, and in return Ali called him a gorilla, an uncle Tom. Ali was most effective from a distance, Frazier liked to get inside and bang that left hook to the body. Frazier would work his way in and Ali kept grabbing him behind the head and pulling him in tight, stifling the inside attack. Its illegal. Ali was never warned or penalized even though frazer says he did it over 100 times in their fights. One ref even slaps Alis glove away when he's holding, but no warning. He lost to Jimmy Young, a friend of mine, but was given the decision. I knew he would lose to spinks the first time. I made money. Ali took him too lightly and came in out of shape. He dragged Ernie Terrells face along the ropes, damaging his eye. But he was a beautifully gifted boxer. A true showman. A self promoter. A great humanitarian who loved everyone. But he did cheat on his wives. I did not serve in Viet Nam, not old enough so I can't speak for those that did. THEY have a right to be angry with him. But I feel he refused to be drafted for the right reasons. Like all legends, he was larger than life and also flawed. He continued to fight long after he should have retired. Unfortunately, he had a lot of people depending on the money he generated so he kept getting in the ring. I will miss him, but I miss all the greats, Frazier, Norton, Patterson, Terrell, Young, etc.
As for how he would have done vs various other champions, I don't think you can really compare eras. I will say this, Ali trained to go the full 15 rounds. Most of todays heavyweights are gassed after 5. Ali was robbed of his best years.

If you have this picture, its Liston, not frazier.

Rope a dope is the worst move in boxing. Back in the day, they would have disqualified Ali. I believe there was a rule once you had to throw a certain number of punches per round. Its was almost as bad as watching john ruiz fight. (Punch punch hold, punch punch hold.)