The supergroup has been a staple of rock since the mid-60s – there have been great ones, good ones, and ones that should never have gotten out of the rehearsal room. The very best are those with that alchemical x-factor that comes from a combination of fame, glamour, novelty and pure talent. The great supergroups are rarely built to last, but even the ones that crash and burn as soon as they get off the ground can often leave an indelible mark on music history. From AOR big hitters to blues rock superstars, here are the 20 greatest rock supergroups ever