The Red Chord are a good band, but they're far from being really extreme. If you like them though, you might want to check out a little-known Danish (I think) band called Lipid. I'm not sure where you can get mp3s of them (bar p2ps), but their album is available through Copro records (
www.coprorecords.co.uk).
Anyway, most extreme band..I don't know. There's many different types of extreme - heaviness/brutality, technicality, sheer volume, bleakness/sparseness, etc.
My gut choices;
Heaviness/brutality - Cryptopsy (with Lord Worm)
Technicality - Cynic, Atheist, Spastic Ink.
Volume - \m/ MANOWAR! \m/
Bleakness - not sure..Neurosis? Or maybe some black metal band..
Also, Six Feet Under - they are, as a friend of mine once said, officially less metal than Perry Como. They're terrible.