Pretty much single handedly responsible for getting me into metal altogether with their 'Toxicity' release. This is one amazing band, respected by critics and fans alike they create very original and unique music which can be appreciated on many levels as they take influence from all sorts of genres.
I've always had a bit a of a hard on for Serj's vocal style. He can be operatic and he can also bellow the house down if he wants to, one of the most distinctive vocals styles I've ever heard.
I've no doubt the "are they Nu Metal?" debate will come into play very soon, I for one don't see how they can be regarded as such, they simply have too many styles in their music to just be classified as simple nu metal.
Anyway, my personal fave album is their self titled debut, just a very chaotic release and very heavy and aggressive whilst having some crazy ass lyrics and compelling songwriting.