I'm on board for any decade list you want to do. Would we do this on a new thread?
If you want a crash course in the 70's, LMP, I'd recommend all Robert Altman from that decade (including Nashville, The Long Goodbye, Brewster McCloud, McCabe & Mrs. Miller, M*A*S*H), all Coppola (in addition to The Godfathers you'd want The Conversation, Apocalypse Now), the fantastic Bertolucci (Last Tango In Paris , The Conformist, 1900) and the work of Hal Ashby, who isn't a brilliant director in his own right but made a handful of the decade's more important films (Shampoo, Coming Home, Harold & Maude, The Last Detail, Being There, Bound For Glory). And you would also want the William Friedkin two-fer (The French Connection & The Exorcist), and Peter Bogdanovich's pre-crash & burn work (The Last Picture Show, What's Up Doc?, Paper Moon). Under the radar might be the work of Bob Fosse (Cabaret, Lenny, All That Jazz), who was talented enough to win the directing Oscar over Coppola, in a year where Cabaret won 7 awards to Godfather's 3. Most impressive.
Couple I forgot are Sidney Lumet, another guy who didn't have much of a distinct style but made some landmark films (Serpico, Dog Day Afternoon, Network), as well as Alan J Pakula (All the President's Men, The Parallax View, Klute.
And you have some filmmakers who did stuff a bit more commerical but well worth seeing anyway, like George Roy Hill (The Sting, Butch Cassidy & The Sundance Kid, The Great Waldo Pepper, Slaughterhouse Five), and of course the early and brilliant Woody Allen films (Bananas, Sleeper, Love & Death, Annie Hall, Manhattan).
For something a bit more esoteric, I'd check out Antonioni's The Passenger (actually in English, and w/ Jack Nicholson!), and if you want to go the foreign route I'd go right to Andrei Tarkovsky and his brilliant 1970's work (the original Solaris, The Mirror, Stalker). Fellini has a couple great 70's films as well (Amarcord, Roma).
Last but the opposite of least, you'll want too see Malick's first two as well (Badlands, Days of Heaven).
I'm sure you have seen some of these already but I wanted to be thorough. I'm not including any early Lucas or Spielberg as those are kind of obvious, and not really "essential" in film snob terms. I also don't need to tell you about Kubrick or Marty. Should be a pretty good primer.
