If there was one specific match you could point to as the clear indicator that both Triple H and The Rock had true superstar potential, it would probably be their classic battle on this date in history (Aug. 30, 1998) at SummerSlam in a ladder match for the Intercontinental title.
At the time, the two were leaders of their respective stables, with Triple H heading up D-Generation X and Rock the main cat in the Nation of Domination. The former was a babyface group while the latter worked heel.
That slowly started changing, however, as Rock continued to show throughout 1998 that he was one of the best promo guys in the business and the fans who so passionately hated him at first were coming around to his act. This was strongly evidenced in this match, as the crowd at Madison Square Garden showed their approval by wildly cheering for all of Rock's major spots, despite his standing as the heel.
Triple H still received a great deal of cheers and really, they both deserved it. They worked their asses off in this match, never needing to hit ridiculous high spots instead using ladders and chairs in creative ways and mixing in solid psychology. Add in outstanding commentary from Jim Ross and Jerry Lawler and this one had it all.
Actually, Ross's commentary in the final few minutes of this match is some of his best work. He really added a great deal to what was already a classic.
Don't take my word for it. See for yourself by watching the match: