When Fury exposed him in the second fight, he was never the same mentally again. IMO when Wilder started talking about spiked water, his trainer working with Fury, the suit, etc to explain the loss, I knew it was over. He didn't have the mentality to accept that loss and potentially improve. He couldn't believe Fury dominated him in that way and became delusional. The beating he took didn't help either.
It's really hard to judge his prime as he was always a one trick pony and regularly had fights where he struggled even against cherry picked opposition, so he never really had a purple patch where he looked at his best. He struggled to impress in first two defences against Molina and Duhaupas, Szpilka too made him look far from unbeatable prior to the KO. Washington too made him look bad early, sweeping the early rounds, Ortiz in both fights too made him look bad. I guess maybe from Stiverne 2 and Breazeale could be his prime. He did better against Stiverne 2nd time around and the Fury draw could be seen as his best performance against Fury as he went the distance. Though neither versions of those fighters were exactly at their best or even close to it.