Most critics at the French Riviera resort had seen no outright favourite for the money-spinning top prize, which was awarded by the jury.
The jury president, the French actress Isabelle Adjani, announced a special 50th anniversary prize, awarded to Egypt's Youssef Chahine for all his work. The Sweet Hereafter, by Canada's Atom Egoyan, won a runner-up Grand Jury Prize.
Sean Penn was voted best actor for She's so Lovely, by fellow American Nick Cassavetes, and Kathy Burke was voted best actress for Nil by Mouth, by Britain's Gary Oldman.
But the critics weren't impressed. "Quality-wise this year has been very average. Just a few years ago, and even more so 20 years ago, there was a masterpiece a day at the festival," said Michel Ciment, who edits the French film magazine Positif.
"What is serious this year is that there were no major discoveries outside the competition, no one like Wim Wenders for example," Mr Ciment said.
"This year there was too much of an emphasis on stars. There were much better films for the competition than Johnny Depp's The Brave, but organisers wanted him and possibly Marlon Brando so it got in," he said.
Surveys by Screen International and Moving Pictures had put Ang Lee's The Ice Storm, about a New York suburban family, and Curtis Hanson's fast- paced thriller, LA Confidential, in a narrow lead.
Outsiders also tipped by Cannes critics to pick up big prizes included Oldman's partly-autobiographical Nil By Mouth, and Michael Winterbottom's Welcome To Sarajevo, about a television journalist's adoption of a Bosnian child.
Mr Ciment opted for Egoyan's film because it "is very rich and complex, and because it's his seventh film and he's at the stage where he needs a major reward".
Violence in many films - sci-fi in the French director Luc Besson's The Fifth Element, or wife-beating in Oldman's movie, among others - dampened birthday celebrations.