Ali's absolute prime came in late 66 to 1967.
Ali's first two fights with Norton were in 1973. Ali claims that he undertrained the first time when losing, but he still had a very tough return match which he won by split decision.
Ali was still very much the best heavyweight on the planet during the next few years, as demonstrated in beating Frazier and Foreman in 1974 and Frazier again in 1975.
After the 1975 Frazier fight for me seems to be where Ali really started to show decline almost fight by fight. He looked awful against Jimmy Young in 1976 and later that year was very lucky to hold onto his title against Norton in their third fight.
Ali wasn't shot though, just in decline.
In 1978 he lost to Leon Spinks, but later that year regained his title in the rematch. Ali looked in great shape for the rematch and danced well, but his accuracy wasn't too impressive, though he certainly deserved the decision handily.
It was around this time that he started exhibiting signs of the early onset of Parkinsons; when he was tired his speach was more gravelly and less clear and he sometimes had a vacant expression.
He certainly seemed shot for Holmes in 1980, though I do agree that thyroid/weight shedding drugs played a part. He was much sharper and more active for Berbick in 1981, but his reflexes were awful. Despite winning a few rounds Ali did not decerve the decision. But what was most worrying was the difference between Ali's speach in the pre fight press conferences and in the post fight interview......you could hardly understand him after the fight!