Love all of their songs, and in no particular order...
Hallowed Be Thy Name off of Number of the Beast
Fear of the Dark off of Fear of the Dark
The Thin Line Between Love and Hate off of Brave New World
Dance of Death off of Dance of Death
These Colours Don't Run off of A Matter of Life and Death
Stranger In A Strange Land off of Somewhere In Time
Children of the Damned off of Number of the Beast
Still Life off of Piece of Mind
Alexander the Great off of Somewhere in Time
Infinite Dreams off of Seventh Son of a Seventh Son
Its a 3 way tie between, Brave New World, Somewhere in Time, and Dance of Death.