After C Diddy's treachery, I must act quickly. Though Aenima is probably, probably my favorite Tool album, this one could be it too. I'm just going to say it is for the hell of it, since you took Aenima. Either way, this record is awesome.
Tool - Lateralus - 2001
Tool is in my top 3 of favorite bands. They are the best current hard rock group anywhere. I think in a few decades, their stuff will be considered classics as they truly have and are leading us into a new wave of progressive rock (which is their official genre).
At the very least this is Tool's most critically acclaimed album, and is in the top 200 Definitive Albums in the Rock and Roll Hall of Fame at #123. Also, this album has much less annoying interludes than Aenima...man I found some of those annoying.
Oh and for anyone into math a little bit like I am, or once was,
Lateralus will blow you away. The
title track is a sort of homage to the
Fibonnaci sequence, in that the lyrics, time signature, and guitar riffs are all assembled to the primary Fibonnaci sequence (1,1,2,3,5,8,13...) or a certain Fibonacci number (i.e. 987, as the time signatures through the song goes through a cycle of 9/8 to 8/8 to 7/8).
The best track on here, IMO, "Schism," won a Grammy in 2002 for Best Metal Performance, and its well-deserved and is basically the best song to listen to if you are just starting up on Tool. it encapsulates basically everything they do, awesome lyrics, multiple time signatures, and some of the best riffs around. "The Patient," "The Grudge," and "Parabol/Parabola" (another math-driven song) are also irreplaceable Tool songs that I just have to have on my island.