(R) 4 1/2 stars Among the great lessons we"ve learned from Martin Scorsese is that a film doesn"t have to actually have a plot to be great. His masterpiece, Goodfellas, has very little plot; Taxi Driver even less so. It"s a lesson Scorsese might have taken to heart when filming Gangs of New York. His tale of Irish and English gangs clashing on the streets of the city in 1863 is at its most effective when depicting everyday life and customs in the slums; it loses its way somewhat whenever it veers back into the main plot. Young Irish immigrant Amsterdam Villon (Leonardo DiCaprio) hits the streets in search of revenge against Bill the Butcher (Daniel Day-Lewis), the English gang leader who killed Amsterdam"s father. Along the way he deals with an untrustworthy best friend (Henry Thomas) and falls in love with a pickpocket (Cameron Diaz), who happens to be Bill"s former favorite lover. Scorsese is never better than when he is depicting the city he loves. As a portrait of 19th century New York City, the film is at once lush and decrepit, with carefully-edited montages of city life that explore the loathsome underbelly. Some of the best sequences contrast the misery of poverty with the extravagance of the high life, and the pettiness of the gang turf wars with the massive destruction of the Draft Riots. The actors prove to be a match for the backdrop. Day-Lewis in particular delivers a stirring performance as the morally-ambiguous leader who holds the lives of his followers in one hand and severe punishment in the other. Jim Broadbent steals every scene he"s in as political machinist Boss Tweed, and may well earn himself another Best Supporting Actor Oscar. The historical details are exacting, although DiCaprio and Diaz look suspiciously scrubbed and groomed for their street-trash roles. The best parts of the film come in the first half, evoking the "I always wanted to be a wiseguy" parts of Goodfellas. The second half occasionally skips back to the class-conflict issues; David Hemmings shows up periodically as a stuffy aristocrat, and every time he wandered offscreen I kept hoping the camera would follow him instead of cutting back to the latest wrinkle in the Leo/Cameron love story. Gangs of New York is close to being Scorsese"s finest. The love triangle and revenge plots keep it from reaching the status of the likes of Goodfellas, but it"s still one of the year"s best films.