I think Chara's had the best career but I don't really think of him as a "since Lidstrom defenseman", as Lidstrom won the Norris more recently than Chara did, and Chara's only 7 years younger, while Keith is 13 years younger, Weber 15, etc.
I voted for Keith here, and granted his first Norris came before Lidstrom's last as well, but I think his career is separated enough that he feels like an after Lidstrom defenseman. Karlsson has an argument as well, but I like Keith's multiple great playoff runs.
Hedman's gaining ground though, and given he's only 30 and wasn't showing any signs of decline before this recent injury, I suspect he'll end up going down as the best of this lost.