I think most of Maiden's heaviest songs only came about post-1995, after the release of The X Factor. With the definitive exception of Virtual XI, each Maiden album since then featured at least one actually heavy song. I don't think Maiden's earlier years had that big an amount of heavy songs, rather than heavy moments on songs.
"The Prisoner" might be the heaviest of the '80s period. "Losfer Words" is also a very strong contender, as are a number of tracks from Di'Anno's tenure.
From the '90s onwards, we get songs like "Be Quick Or Be Dead", "Sign of the Cross", "Fortunes of War". A number of songs from Brave New World, Dance of Death, The Final Frontier and The Book os Souls also showcase unique moments of extreme heaviness. "The Book of Souls" may feature the heaviest riff in Maiden's career.
It is "Face In the Sand", though, that I consider to be Maiden's heaviest song yet. I don't quite know why, but it feels gigantic; like it's going to crush you completely.
A Matter of Life and Death is definitely their heaviest album, followed closely by The X Factor. Dance of Death can also be very heavy at times ("Face In the Sand" as the obvious example), but that one is more due to the production itself than the subject matter of the lyrics.