Rap. Rock. Two great tastes that usually don't taste great together. It always seems like a good idea: Take some great rock 'n' roll group, pair them with a hot rapper, what could go wrong? A lot it turns out. But that's not to say the experiment is always a failure. From the Red Hot Chili Peppers with LL Cool J to Onyx mixing it up with Biohazard (yes, Onyx and Biohazard), these are the 20 Best Rap Rock Collaborations.