The Melvins will melt your face off. Sludgy, dark music with a pretty decent sense of humor. Everyone starts with Houdini, so I'd say start with Houdini. Their stuff is all over the map, but I assure you it's all connected to being sludgy and dark. I forget which album Kurt Cobain produced, but I think this is the one. They really sound nothing like Nirvana.
They're the greatest live band I've ever seen, yet I have no desire to listen to their albums ever. I don't really like heavy stuff honestly, but I can't deny the greatness and the intensity of their live performance. I have my friend to thank for dragging me along.