Amitabh Bachchan's flaws started occurring in 1988 & 1989 with Flops barring Shahenshah which was a Hit. However, he still had stardom and evidence is shown in terms of Record Opener & Bumper Opener. Even Agneepath was a Flop but had a Bumper Opener and was the fourth highest-grossing movie below Aaj Ka Arjun (Record Opener) Followed by that, Hum was a Hit and Record Opener. His craze and stardom was there but after he came out retirement he suffered a lot. ABCL was in debt and Bachchan had to pay back the money that he owed which forced him to bad films like Mrityudaata, Lal Baadshah, Kohram, other movies which all tanked at the Box Office. Bade Miyan Chote Miyan was a Bumper Opener but Bachchan's marketing value was had faded. It was part of the reason why he started to do character roles in the 00s but he was also bankrupt at that time. That's when Yash Chopra whom he wasn't on terms with gave him a role in Mohbabatein.
As far as top three goes, I would say the early 90s but as mentioned, he suffered due to his production company ABCL.