There is a case to be made for Michael Mann as America's most unlucky director. The Keep is one of the 80s' most abstract and interesting horror movies. It was made in the UK but never released here. With Manhunter he kick-started the Thomas Harris cult, but saw his masterly picture gather dust at the UK distributors for three years, and its thunder stolen by the inferior Silence of the Lambs. Worst, the astounding existential thriller Thief (1981) gathered the best performance James Caan has ever given but was crudely retitled (Violent Streets, if you must) and barely seen. It stands as one of the finest American films of the past 15 years.

The uncharacteristic, exhilarating but slight Last of the Mohicans made Mann bankable at last, and three years on he's back at what he does best. Heat is a slow, sprawling existential thriller par excellence, pilfering the best elements of both Thief and Manhunter and wedding them in an extraordinary portrait of a modern American city.

Thrice-married Los Angeles police detective Vincent Hanna (Al Pacino) is called to investigate the aftermath of a particularly brutal armoured car hijack by a highly gifted gang of heavily armed thieves. Not much time is wasted in locating one of the crew responsible, and through him ruthless master crim Neil McCauley (Robert De Niro). In McCauley, Hanna recognises a kindred spirit, a dedicated professional, and will stop at nothing to bring the man down. For his part, McCauley is spiritual cousin to Caan's thief, the same cool, suppressed emotionalism that can have sudden, shocking consequences. His credo is to have nothing in his life that he can't walk away from in 30 seconds.

In bald terms there is nothing unusual about the picture. It's a cop movie staple, but in the hands of a consummate writer/director like Mann it sings. Plus, of course, casting De Niro and Pacino opposite one another for the first time (The Godfather II doesn't count) raises expectations that, for once, are fulfilled in spades.

Pacino is excellent with the bigger, flashier role. Hanna is wired, edgy, telling his wife (Diane Venora) that he needs his angst to stay sharp, to stay on top. Like Will Graham in Manhunter, Hanna has a criminal mind, able to see through the eyes of his prey. He is no loose cannon, merely a man who pretends to be. Pacino is an actor capable of wonderful things, of course, but also one walking a very fine line. The distance between, say, Carlito's Way and Revolution can be a narrow one. Mann keeps him in check and the results are impressive.

But if his soulmate is good, then De Niro is magnificent. This is his most compelling performance certainly since what was effectively an extended cameo for Scorsese's Goodfellas (1990). Gone are the grandstanding tics and smirks that have passed for talent in the last lean decade, back the great actor controlling a scene simply with his eyes. De Niro is best when he (apparently) does nothing, and in doing nothing here he steals the picture. We haven't seen him this poised, this elegant in charge of a movie since the scarifying King of Comedy a dozen years ago.

Mann's genius is to keep the two in close proximity but apart for all but ten or so minutes of the almost three hour running time. The movie's pivot, therefore, is the electrifying coffee shop meet to compare professional notes. It's for the classical shooting, its calmness that the scene pulsates even more than the handful of vivid shoot-outs that punctuate the picture. It'll become a classic of 90s cinema.

Against these two it is testimony to the supporting cast that they make such a strong impression. Val Kilmer as De Niro's right-hand man has never been better than he is here, and Tom Sizemore (so good recently in Devil in a Blue Dress) is superb as a fellow gang member. Jon Voight, as the spectacularly jaundiced-looking fence/fixer Nate, almost makes up for all the scenery chewing that has passed for his career for too long now. Tom Noonan and Ted Levine (the psycho-sickos of Manhunter and Silence, respectively), Venora, Wes Studi as a cop, and Ashley Judd as Kilmer's wife, are all impressive. Only Amy Breeneman (from TV's NYPD Blue) as De Niro's love interest fails to score.

Other than the two heavyweights, the real star is Mann's Los Angeles. He is arguably the best director of architecture currently working, and gives this most anonymous of North American cities a depth seldom seen on screen. Diving through deserted, crepuscular streets or skirting above them in a police helicopter, Dante Spinotti's camerawork is formal, almost classical. It can do calm (those surveillances) and action (the film is structured by three big set-pieces) with equal assurance. Mann's visual signature is all over the film in its strong primary colours, its sumptuous use of the widescreen frame; Mann is that rare thing these days, a powerful film-maker for the big screen.

There is a potent score from Elliot Goldenthal, intercut with Mann's usual, uncanny ear for found music. Thus he employs such diverse talents as Ambient wizard Brian Eno, the classical strains of Ligeti, German avant-noisers Einsturzende Neubauten and (memorably) jazz guitarist Terje Rypdal to superb effect. Mann, like Scorsese, is a director who uses music for emphasis, not to shift soundtracks.

It can be argued that the climatic shoot-out at LAX is a disappointment (certainly the final line is a clunker) but after 171 minutes of fat free, pile-driving cinema of this quality then a truly resonant payoff would be hard to conjure. That aside this is three hours of the best cinema mainstream Hollywood has offered us in many a year and finally cements at the box office what some have realised for a long time - that Michael Mann is one of the half dozen very finest film-makers in America.