Rookie Tim hadn't learned how to pass out of the double team. When they did double him, he turned the ball over a bit too often. But he had David on the team and the defenses couldn't afford to double him as much.
As a rookie, he also didn't get to the FT line nearly as much as he did a couple of years later, and he shot FT's very poorly when he did get to the line. He was a very good defender, but not nearly as good as he would become.
If you want to talk about the '99-2000 Tim Duncan, then yes he would be the best player in the league. I don't know about rookie Tim, and I don't want to have to think that hard, or split hairs. By 2000, he was impossible to handle without double teaming most of the time. He had that little bank shot from out 15-17 feet that was deadly, and almost unguardable. Back to the basket or facing the basket, if they didn't bring the double he cut defenders to shreds (and shot something like 70% from around the basket.) And he owned the paint on defense.
I don't care how other teams are playing. He would terrorize the paint. With him anchoring the middle, guys like Forbes are worth twice as much. He would cover for more of their defensive mistakes, and give them a LOT more uncontested 3P shots.
So yes - he would be.