(FORTUNE Magazine) – His cable company pumps music videos to more than 21 million homes and delivers fast Internet service to four million users, making Roberts, 44, a very important man to the MTV set. (Eminem even raps about watching Comcast in "Criminal.") But what makes this CEO really powerful is the growing clout he wields over grownups such as AOL's Dick Parsons (No. 24), Disney's Michael Eisner, and Viacom's Mel Karmazin: Roberts is seeking volume discounts on the programming he buys from those media moguls. Given Comcast's incredible market strength, many industry insiders are betting Roberts gets his way. A former squash All-American, Roberts is relentless when he puts his mind to something. He never flagged in his bid to acquire AT&T's cable assets, for example, a $51 billion transaction that turned No. 3 Comcast into the largest cable operator in the country. And because his family has 33% voting control, Roberts has something many of his rivals don't: job security. --S.N.M.