Nobody is saying that. And that's not how the draft happens anyway. GMs aren't faced with the choice of 5 good defensemen and a mediocre center. They're choosing between a good center and a good defenseman. There is absolutely no objective way of determining whether Dalibor Dvorsky is a "better" center than David Reinbacher is a defenseman. But pretty much everyone universally agrees that they're roughly as good at their respective positions as the other is. THAT'S the decision GMs have to make.
"Best" is an objective term that denotes quality. If you have a scale from 1-10, 1 being worst and 10 being best, then the "best" player available would be the player highest on the scale who can still be chosen. But that's not how teams work because there's no way of determining whether a good playmaker is better than an equally good shooter (for example). They chose the player they "like" the most of the remaining choices. And that's totally subjective and could be based on a whole bunch of reasons that have nothing to do with hockey skill. Every single year we hear about a kid who would have gone higher if not for "character issues". Or a goalie with starter upside who gets drafted later than a middle six winger because "goalies are voodoo". GMs also routinely choose players with a higher floor but a lower ceiling, not because they're "better" but because they're safer. These aren't objective talent evaluations, they're preferences.
Take the Seider draft as an example. Literally nobody (other than Yzerman) thought he was going to get picked 6th. So one of two things must be true. Either Yzerman is the only person in hockey who is capable of accurately determining who the "best" player is, or Yzerman took the player he liked the most, who also happened to play a position of need, and that player turned out to be the best player after the fact. Zadina is the same example in reverse. EVERYONE agreed he was the best player for Detroit to take based on their evaluations. But clearly he wasn't. So either everyone has no idea what the "best" player looks like, or there is absolutely no way of telling on draft day. In either case, it would be completely idiotic to base your draft strategy on it.