According to Wikipedia many critics considered Orson Welles' three-minute, twenty-second tracking shot in TOUCH OF EVIL as one of the greatest long takes in cinematic history, especially considering that it's the opening scene.
Here's the synopsis.
On the U.S.-Mexico border, a man plants a time bomb in a car. A man and woman enter the vehicle and make a slow journey through town to the U.S. border. Newlyweds Miguel "Mike" Vargas (Charlton Heston) and Susie (Janet Leigh) pass the car several times on foot. The car crosses the border, then explodes, killing the occupants.
To see the opening scene of TOUCH OF EVIL PLEASE go to the following URL