My personal preference is Potvin. I'm not trying to diminish Lidstrom's dominance of this past decade and all his accomplishments, but the fact remains that he won his Norris Trophies in a relatively weak era for defensemen (his best competition has been Pronger, Chara, Boyle, Gonchar, Zubov, etc.). Most of the dominant defenders of the 90s had retired or were past their prime by the time that Lidstrom started dominating the league.
Potvin, on the other hand, was competing against Park, Robinson, Savard, Lapointe, Salming, etc. Therefore, I don't like to just use the comparison of Lidstrom's six Norris Trophies to Potvin's three, or Lidstroms eight first-all-star teams versus Potvin's five. Awards don't tell the whole story.
Personally, I have Potvin as the 5th greatest defenseman of all time (behind the obvious top 4), and have Lidstrom, Robinson, and Kelly (in some order) in the 6th, 7th, and 8th spots.