Little Feat - their entire output with Lowell George
Randy Newman - Sail Away, Good Ol Boys, Little Criminals
Paul Simon - three of his best solo albums came in the 70s (although I would argue that he has maintained a consistently high level of songwriting to current day)
Nick Drake, Big Star, Miles Davis (Bitches Brew and others), XTC...
Oh... and how about Stevie Wonder?
I love all of Paul Simon's solo work from the 70s, as well as the early work of both XTC and Elvis Costello.
I would add lots of great music from Fleetwood Mac, The Eagles, Steely Dan, Steve Miller Band, Rush, Roxy Music, Boston, Genesis, The Doobie Brothers, Heart, Journey, Dire Straits, The Kinks, Black Sabbath, and The Allman Brothers.