To me, the best all around defensemen in Sabres history were Mike Ramsey and Jim Schoenfeld. They scored, they assisted, they hit, and they defended like few others.
Bill Hajt was solid offensively, but was maybe the best defender we ever had. His +320 career is #24 amongst all NHL players in history.
You say not Housley, yet he is the best comparable to Dahlin, and his PPG well outpaced Dahlin's career so far.
If you want to say Dahlin appears to be on his way to being one of the top 10 defenseman in Sabres history, I can get behind that. However, if you want to declare a 22 year old defenseman in his 5th year, who has not won anything yet, the best defenseman in Sabres history? Not a chance.