I second Michael Mann as being a great director. Here are some directors that I think are incredible and some of their films (and many of their films rank among some of the best films ever made):
Dario Argento - Deep Red, Suspiria, Tenebre
Andrei Tarkovsky - Stalker, The Mirror, Nostalghia, The Sacrifice, Andrei Rublev, Solaris (the original)
Ingmar Bergman - Through A Glass Darkly, Scenes From A Marriage, The Silence, Winter Light, Persona
Akira Kurosawa - Ran, Kagemusha, Yojimbo, Sanjuro, Seven Samurai, Red Beard
Peter Greenaway - The Thief, The Cook, His Wife & Her Lover, The Belly of An Architect
Samuel Fuller - Shock Corridor
Peter Weir - The Last Wave, Picnic At Hanging Rock
Kihachi Okamoto - Sword of Doom
Jules Dassin - Night and the City, Rififi
Roman Polanski - Knife In the Water, Rosemary's Baby
Henry Georges Clouzot - Le Corbeu, Wages of Fear