They're all pretty fucking awesome. Kreator's first five are UNFUCKINGBEATABLE, but then they took a total nosedive. Early Destruction (apart from Infernal Overkill) honestly doesn't do much for me, but All Hell Breaks Loose is excellent, and The Antichrist is one of the all-time great thrash albums. And of course, Sodom is insanely consistent and badass. I just picked up M-16, it's right up there with Agent Orange and Persecution Mania, if not surpassing it.
Then there's all those other awesome bands, Tankard and Exumer in particular. This is hard. I'm just gonna say Kreator on the basis of Coma of Fucking Souls and Pleasure To Fucking Kill, but that's no sure thing. The new one looks promising though.