I got to go with Kenny Clark, he's dominated the run game as a 4-2 NT... as the Packers are 7th in defensive yards push rush at 3.8 and helped keep Martinez clean, who's also stepped up and is my honorable mention.
And Damarious Randall has to be mentioned since being sent to the locker room, he's stepped it up... getting turnovers and not hearing him complain or coaches taking off the field to talk with him no more.
Also maybe Davante Adams, while his stat average seem to be on pace to be the same as last year, this year he's been able to keep his pace up even without Rodgers throwing to him and while he's getting more attention from defenses (#1 CB following him)... and becoming Hundley's favorite target.
It's hard for me to say any rookie is the most improved, as they weren't hear last year... though Jamaal Williams has looked a lot better than earlier this year... while his average is still bad, I'd be interested in his yards after contact stat as he seems to get touched early but keeps going and picks up extra yardage.
Same with new FAs, as they weren't hear last year, but OG Jahri Evans has seemed to be a very good signing, while he's no TJ Lang, he's not that far behind and a lot cheaper.