A teen's definition of cool swings with every trip to the mall. What's hot today might be on the clearance shelf tomorrow. Each year, Teenage Research Unlimited asks some 2,000 teens to identify the three "coolest" brands of the moment, and 1998 brought a few surprises. Nike maintained its number-one ranking, but only 38 percent of teens, compared to 52 percent in 1997, named the brand in their top three-a drop-off of nearly 40 percent. Adidas scored high points, leaping from sixth to second place within the last year. Retailers Old Navy and Abercrombie & Fitch entered the list in '98, as did Sprite. Mountain Dew, hot with snowboarders and skaters, returned in 1998 after an absence in 1997.