I think the obvious answer here (assuming I understand the question correctly) is The X Factor. That was not a popular album when it came out, and I imagine at the time many people perceived it as a blip on the band's fade into obscurity. The two albums before it were a clear step down from the 80s material, Bruce was gone, Adrian had been gone, the production was flat and the new singer was a bit of an odd fit.
But I think when you look back at The X Factor 25 years on, I think it's the album you have to point out as the beginning of the prog Iron Maiden of the reunion era, and when you listen to it without worrying that you'll never hear the band with Bruce and Adrian again, or that all their future music will forever be this dark and depressing, there is actually a ton of great material on there.