The Tonight Show was the late-night champ for the fifth week in a row last week, drawing 4.20 million viewers. But it all amounted to something of a pyrrhic victory, inasmuch as its ratings were down 26 percent from the same week a year ago. Closing the gap in second place and coming in with 3.66 million viewers was ABC's Nightline , whose ratings were up 5 percent from a year ago. CBS's Late Show with David Letterman drew 3.19 million viewers, down 8 percent from last year. Among 18-49-year-old viewers, the results were even tighter. Tonight led with 1.73 million viewers; Nightline had 1.55 million; and Late Show, 1.46 million. Allowing for statistical error given the tiny percentage separating the three shows, the results amounted to a three-way tie.