IMO, Tool should never be considered "mainstream." Maybe this is a bit of elitism emerging in me, but if you were to actually look into Tool songs and their meanings, etc, you would see how they are much deeper than "mainstream" music.
As far as my favorite genres: metal, classic rock, jazz, fusion, classical, etc.
Some of my favorite bands: Metallica, Led Zep, Tool, A Perfect Circle, Meshuggah, Opeth, Primus, Deftones, Finger Eleven,
Isis, Jamiroquai, The Mars Volta, NIN, Rammstein, Rage Against the Machine, Rob Zombie, Rush, and Tenacious D.
At least Opeth isn't constant screaming, and the singer has one hell of a set of pipes!