The last time the Leafs had a Number 1 D was Rielly. 5th and 11th in Norris voting , All Star 3 times.
He's not that player right now, but that shouldn't cloud your vision that if you are voted 5th best D in the league, you are clearly viewed as a Top 30/32 D, and Number 1.
Now, I think what's actually being alluded to here, is an Elite #1 D, which frankly is pretty rare, and usually only a handful in the league at any one time. Rielly has never been close to that.
McCabe/Kaberle had a nice run as a first pairing that was of high quality, but neither was an Elite #1D.