IMO Jack won't be available when GB picks, but he's rated as one of the top LBs and from what little I've seen has good speed and aggressiveness. I've read people call him a much better version of Shaq Thompson, who IIRC went R1 to Carolina last year.
I like Kentrell Brothers, which might be a kiss of death. I think he has enough sideline-to-sideline speed to do well in certain packages, and the ability to play coverage. I think there's a need for a player like that.