While I enjoy songs on Opiate & Undertow, Aenima & Lateralus are definitely the band's most special albums to me. Without a doubt my two favorite albums from any band, really. I think they started becoming a unique band with Undertow, but didn't quite reach it; then with Aenima they suddenly became perfect. Then Lateralus came along, which proved to be far more 'sophisticated', perhaps.

I mostly judge the albums by the number of Toolgasms in them. Aenima is full of them(Stinkfist, h., 46&2, jimmy, Pushit, Third Eye), Lateralus has many as well, but they're not as strong as Aenima's, with the exception of Schism and Reflection. 10,000 Days has them too, but they're only really strong in Rosetta and The Pot.