Tim Duncan is listed as a power forward (Basketball-reference lists him as a F-C, but most people talk about him as a forward). He is, by any standard, one of the best basketball players of all-time, but his position often gives him the status of greatest ever at his position.
But I don't think Duncan is a forward. I think he's a center.
When Duncan came into the league, there were about 5 teams on which he would have been assigned to play forward; on any other, he'd be a center. Well, he got drafted by one of those 5 teams, so he "became" a forward.
But on Sunday against the Mavs, Duncan was often on the court with four other guards or swingmen. Some of the time the second tallest player on the floor was Michael Finley. He was the only player posting up. He was the only player with his back to the basket. He was the only Spur getting any production near the basket, with rebounds and with strong to-the-basket postup moves.
Much of the time, there is no way to call any other Spur on the court a center.
It could be that in the current evolutionary state of basketball, there is no real "center." That's fine--then call Duncan a forward. But when comparing him to all-time forwards, keep that in mind. If we called Duncan a center, we would consider him a top-10 player at his position. At the highest, I'd rank him 6th (behind Kareem, Wilt, Russell, Shaq, and Hakeem). There's also Mikan, Reed, and Cowens to consider. Moses Malone, too, though I've always thought of him as a forward, not a center (I'm probably completely wrong on that. Basketball-reference lists him as C-F).
Duncan's game, though, is a center's game.