I have to say the 60s followed by the 90s.
1969 The Wild Bunch, Butch Cassidy and the Sundance Kid, Z
1968 2001 A Space Odyssey, The Lion in Winter, Hour of the Wolf
1967 In the Heat of the Night, Samurai Rebellion, Cool Hand Luke, The Graduate, Marat/Sade
1966 The Good, The Bad, and the Ugly, Andrei Rublev, Sword of Doom, Who's Afraid of Virginia Woolf?, The Battle of Algiers, Closely Watched Trains, Persona
1965 Doctor Zhivago, The Sound of Music, The Shop on Main Street, For A Few Dollars More, The Flight of the Phoenix, Juliet of the Spirits
1964 Dr. Strangelove, A Fistful of Dollars, Zulu, Zorba the Greek
1963 8 1/2, High and Low, The Leopard, The Great Escape
1962 Lawrence of Arabia, Harakiri, The Longest Day, The Miracle Worker, Sanjuro
1961 Yojimbo, Through a Glass Darkly
1960 La Dolce Vita, Spartacus, The Virgin Spring, Inherit the Wind
You have Fellini, Kubrick, Bergman, Kurosawa, Lean, Tarkovsky, and Leone at the height of their powers in the 60s, although the single best year for film has to be 1957.
1957 Wild Strawberries, The Seventh Seal, Paths of Glory, Nights of Cabiria, Throne of Blood, The Bridge on the River Kwai, 12 Angry Men, Witness For the Prosecution, Kanal