Eyehategod and Buzzov•en. They had an important role in determing the characteristics of the genre.
Their music could be a perfect example to introduce the mix of desperate anger, disgust and bitterness. After the heavy sludge masterpiece self-titled debut album they returned with another one, and by "Our Problem" they successfully exceeded themselves. The doom influences decreased, but their songs didn't lose from their heaviness. They became more intese and more complex too even in the case of extented song lenght. The very noisy sound remained as an important feature, but "Our Problem" is also quite groovy and riff-centric compared to the first album. It's like a heavy, chaotic rampage, probably the angriest, and one of the most enjoyable sludge metal albums. The iconic insane, inarticulated screaming vocal style of Johnny Morrow had an important role in forming this impression. Real musickness from the impoverished metal scene of the late '90s!