Slayer, all the way. Old metallica may have influenced the mainstream metal and virtually created the Nu Metal which i find the bane of my existance, with kids going around talking abou t deftones and how much they love metal, and i concede that one of my favorite albums by any artist is Ride the Lightening, but the influence stops at mainstream and rhythm techniques, which suck anyway, but Slayer definately was a steadfast pioneer and a molding force that thrash and extreme metal, which is definately the most technically skillled genre there is. My band is very strongly influenced by Slayer and their riff building techniques, which like Tom Morello and unlike Kirk Hammet, dont exactly follow the traditional music theory, forcing kerry king and his followers, myself included, to constantly think outside the box, and reshape the face of extreme music.