Quentin Tarantino - While I think Tarantino is a fine director, I think he is a far superior writer. Reservoir Dogs, True Romance, Pulp Fiction, Natural Born Killers, From Dusk Till Dawn, Jackie Brown, Kill Bill, Inglorious Basterds, and others are worth studying for their story structure, dialogue, and characterization.
Woody Allen - Unlike other filmmakers who wrote the scripts for movies that they directed (Sylvester Stallone, George Lucas, M. Night Shyamalan), Allen has been consistently providing stories and characters that have been surprisingly nuanced, entertaining, and thought provoking. Measured only on box office success, he might not be the biggest moneymaker, and his days of pop culture relevance may have passed their peak, but he continues to add to his impressive filmography. He has written many great films. My personal faves include Sleeper, Annie Hall, Zelig, Hannah and Her Sisters, Deconstructing Harry, and Celebrity.
John Hughes - The man wrote some of the most memorable movies of the 1980s and early 1990s: National Lampoon's Vacation, Sixteen Candles, The Breakfast Club, Weird Science, Pretty in Pink, Some Kind of Wonderful, Planes, Trains, and Automobiles, Home Alone, and what I consider to be his masterpiece, Ferris Bueller's Day Off.
Billy Wilder - A true legend, I love all his movies, including Double Indemnity, Sunset Boulevard, Sabrina, The Seven Year Itch, Some Like It Hot, The Apartment, and others.
William Goldman - Writer of the classic chronicle of Hollywood filmwriting, Adventures in the Screen Trade, Goldman gave us Butch Cassidy and the Sundance Kid, The Stepford Wives, All the President's Men, Marathon Man, The Princess Bride, Misery, The Chamber, and more. I hated his adaptation of Maverick, but he has still had a great career.
Joel and Ethan Coen - With a string of great movies, this brother team has given us some of the finest motion pictures ever: Blood Simple, Raising Arizona, Miller's Crossing, Barton Fink, The Hudsucker Proxy, Fargo, The Big Lebowski, O Brother, Where Art Thou?, No Country for Old Men, and Burn After Reading.
Christopher Nolan - At times recently I thinking he might be getting overhyped, but I still think there is greatness in this man, not just as a director but as a storyteller too. For evidence, check out his films: Memento, Batman Begins, The Prestige, The Dark Knight, and Inception.
John Patrick Shanley - He is better known as a writer of stageplays, but his Hollywood career is pretty notable too. Although he had dud with Joe vs. the Volcano and Congo, his other movies have been pretty great: Moonstruck, The January Man, Alive, and Doubt.
Cameron Crowe - What a great variety of films Mr. Crowe has written! They include Fast Times at Ridgemont High, Say Anything, Singles, Almost Famous, Vanilla Sky, and Elizabethtown. Oh, and Jerry Maguire, which I think is a bit overrated but many people love.
David Mamet - Another playwright who brought his talents successfully to the big screen. His credits include The Postman Always Rings Twice, The Verdict, The Untouchables, We're No Angels, Hoffa, The Edge, The Spanish Prisoner, Wag the Dog, Ronin, Hannibal, Heist, Glengarry Glen Ross, American Buffalo, and more.
Feel free to let me know any of your own personal favorites that I did not mention.