Both Sabbath and Aerosmith fucked it up after 6 awesome albums, BTW. I have a soft spot for Tunnel of Love, but Springsteen would probably fit there as well.
Haven't heard Cultosaurus Erectus for a long time, but BOC might be a runner up. Midnight Oil definitely woud be (debut to BSM, probably). And Steely Dan (debut to Gaucho, probably).
But I'd probably pick Symph X as mentioned or Elton, Tumbleweed -> Captain.