I can't really rank them, they're all so good that the quality is all there, and the differences in overall quality are so minor that no way the album I'd put last is six albums behind the first one.
I mean, if I put No Prayer well below Piece of Mind, Seventh Son and 666 it's a fair ranking, putting, say, Final Frontier behind Book of Souls, Dance of Death and Senjutsu is not fair.
Maybe I could sum up what I love in few words...
BNW = great comeback, maybe a bit safe
DOD = great personal memories, insanely high points, a couple of throwaway tracks
Matter = grew over time, all tracks are great, can't complain but after having DOD so much the difference is there
FF = same template of songs as Matter, overall enjoyable but more of the same
BoS = awesome and daring, felt a bit fresh, some utter masterpieces and some lesser songs
Senjutsu = once again, it suffers from 6 years of having loved the previous one but it's a grower. Very enjoyable experience. The "same ol' tricks" however are more evident than ever.