Best hard rock/metal band of all time; based on what popularity, record sales, ticket sales? There are too many factors such as age, musical influence, and personal opinion to even debate this topic. Over the years, hard rock/metal bands have stood the test of time, sold millions of albums, and have created a solid fan base.
In no particular order:
Metal: Iron Maiden, Metallica, Black Sabbath, Ozzy Osbourne, Motorhead, Judas Priest (No Homo)
Hard Rock: AC/DC, Motley Crue, Van Halen, KISS, Guns N Roses, Def Leppard