Just starting with the 1980s, I hardly think Jim Cameron's output of The Terminator, Aliens, and The Abyss (not even counting Piranha II: The Spawning) could reasonably be considered the "best" director of that decade. You'd have to argue pretty hard.
Off the top of my head, Woody Allen made about ten films in that period, including Crimes & Misdemeanors, Hannah and Her Sisters, The Purple Rose of Cairo, and Zelig (among others). Rob Reiner's career started off like gangbusters with This is Spinal Tap, The Sure Thing, The Princess Bride, Stand by Me and When Harry Met Sally.... Peter Weir had a great decade starting with Gallipoli, ending with Dead Poets Society and sandwiching Witness, The Mosquito Coast and The Year of Living Dangerously. Marty Scorsese had Raging Bull, The King of Comedy, After Hours, and The Last Temptation of Christ (plus The Color of Money). Spielberg had an "OK" run, I think you'll admit, with Raiders of the Lost Ark and the first two Indiana Jones sequels, E.T., The Color Purple, Empire of the Sun and Always. Even John Carpenter I'd argue had a better overall decade than Cameron, with The Thing, Starman, Big Trouble in Little China, Escape from New York, Christine, They Live, The Fog. Barry Levinson started out very fine with Diner, Tin Men, The Natural, Good Morning Vietnam and Rain Man. Hell, even John Hughes with Sixteen Candles, The Breakfast Club, Ferris Bueller's Day Off, Planes Trains and Automobiles, Uncle Buck, Weird Science and She's Having a Baby.
Those are not obscure names, and all of them had more than only three films in the 1980s. And that's not even getting into filmmakers working in foreign languages or the more arthouse fare.
But you can pick Cameron if you want.
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"Film is a disease. When it infects your bloodstream it takes over as the number one hormone. It bosses the enzymes, directs the pineal gland, plays Iago to your psyche. As with heroin, the antidote to Film is more Film." - Frank Capra