Though Undertow and Aenima are really good, Opiate doesen't have any throw-away tracks on it.  For instance, Aenima has 15 tracks, but only 9 real songs (the 6 other tracks are pretty much noises and crap).  That's not the only reason Opiate is excellent (in other words, because it doesen't have any crappy songs), they've got a really cool blend of sounds ("Sweat" sounds like a cross between Nirvana and Metallica), and they're really good to listen to if you want to get pumped up, etc.  If you're a TOOL fan, you should definetly get this.