It's gotta be Francis Ford Coppola. His drought is going on 30+ years. Nothing he's made since Apocalypse Now even comes close to his 70's work.
Crazy to think you can make so many great movies of all time in the 70's then basically do nothing since. Did his experience on Apoc Now make him lose the plot permanently?
To echo the Rodent, Paul Verhoven has been a huge disappointment. Robocop, Total Recall and Starship Troopers all in a 10 year slot then nothing.
I recently made a thread about Ridley Scott on this very issue, he just seems to be making stinkers on the back of some truly great films, Alien and Blade Runner early on and some greats in the early 2000s with Gladiator, Black Hawk Down and Kingdom of Heaven then 10 years of crap since.
John Mctiernan, Die Hard, Predator, Hunt for Red October, Die Hard 3 then his career basically pitters out to nothing.
Spielberg, Saving Private Ryan, Minority Report, Catch Me if You Can late 90's/early 2000's then basically average movies with a few real stinkers since.
Clint Eastwood is perhaps becoming another, good work as a Director throughout his career to early 2000's then nothing very good since. Although he is 84 now so maybe time to retire Clint.