It's Karlsson and to be honest it's not even close. Alfredsson was a marvelous player but he was never at any point that I can recall considered arguably the best player in the league. Karlsson is one of the most stellar talents to play on the blueline in God knows how long. AND he doesn't have the benefit of playing in the new rules post 04-05 lockout. AND he doesn't have the benefit of playing with a couple of guys like Spezza and Heatley in their prime. Also, a lot of the things Alfredsson is credited for didn't come until late in his career here. The 2007 playoffs was probably the single deciding factor in management not trading him away. People often forget how often Alfredsson was mentioned as being an underachiever for this team when it mattered.
IMO, Alfie was robbed by Lecavalier and Niedermayer for the Hart and Conn Smythe, respectively. Alfie was simply underrated, and came into the league without any bells and whistles as a late round pick (overager) who came in and won the Calder to the smallest fanfare known to NHL fans.