For sheer inventiveness and diversity, I've gotta go with FII. Listen to the variation of the drum sounds between the songs, the experimentation with compression on certain instruments, the clarity of Derek's keys, John's spectrum of guitar tones. Sonically, it has left the biggest impression on me, but that's also possibly due to some of the songs being a bit of a departure from normal DT fare. There was some stuff going on with those songs that allowed for very wide interpretation in the mix as well - variety that wasn't really possible with a concept album like SFAM or the rather two-dimensional approach of ToT. I think the same thing can be said of 8VM - variety of genres and song styles allowed for a lot of experimentation in the production and mixing, but I think this was employed better on FII.