I don't consider guys like Yandle #1 defensemen, to me you have to be a good scorer from the blueline (30+ points) while also being able to play on the top pairing and get the toughest matchups defensively.
The question is, as the OP put it, what the definition of a #1 D-man is. And if I know HF, there will be arguing to no end that definition with no consensus, usually going between these following viewpoints:
-There are 30 teams, hence 30 #1 D-men in the league, so the top 30 D-men are #1 D-men
-#1 means first on the depth chart per team, so by definition teams can only have one #1. The other guy would be #2, even if he would be #1 on a different team.
-#1 D-men are guys who could be considered a #1 on a Cup-contending team
-#1 D-men are Norris contenders...there are actually relatively few #1 D-men in the league. They are the elite of the elite.
And that's not even touching the "offense vs defense" arguments.