Nik Lidstrom. Probably biased. All well.
Bobby Orr, Ray Bourqe, and Paul Coffey are my top 3
As to why I say there isn't discussion as to who is the best, it's because in my experiences Orr is always considered the best defenceman. I'm sure there is one hell of an argument for Lidstrom, though.
Well, probably my all time favorite defenseman was (Texas born! ) Brian Leetch, but he is not the greatest ever. He was so fun to watch though how offensively explosive he was. Some of the records Coffey has blow me away: the goals in season; points in a game. Bourque leads both most goals and assists in a defenseman's career.
Al MacInnis put up astounding numbers, and the Gretzky of American (well, at least Midwest) hockey is Housley, and if anyone knows some of the legend of him - straight from high school into the NHL and what he did in high school (something that might never be done again) - he could run the whole show like a quarterback - you'd be surprised.
Orr put up crazy good numbers for a d-man. Wonder what he'd do in the league today.
For those interested: http://couchpotatohockey.com/Records/Reg_Season_Ind_Records/Scoring_by_Defenseman.asp