The Book of Souls, for a couple reasons.
The first and most important being the incredibly dull and disappointing title track of Senjutsu. I will never get over the disappointment of first listening to the album, and listening through that god awful title track, waiting for it to get interesting and pick up momentum. I guess I expected another If Eternity Should Fail. So that first song started me off with a sour taste of Senjutsu. By far my least favorite Iron Maiden song.
Secondly, I feel that TBoS has more great songs (Eternity, The Great Unknown, The Red and the Black, When the River Runs Deep, THE BOOK OF SOULS - which is probably my favorite reunion era song). TBoS does have some filler though, especially on side B.
I feel that Senjutsu has a lot of filler as well, including Days of Future Past, Darkest Hour, and Death of the Celts. I will give credit where credit is due, however. I feel that sonically Senjutsu is a better album. And I really enjoy many of the songs on it as well, especially TWOTW, Lost in a Lost World, The Time Machine, and Hell On Earth.
Also the album artwork for TBoS is better too. Couldn’t they have used Senjutsu’s inner sleeve art for the cover instead? Psshhh.