I'd honestly say Odell Beckham, Jr. He really strikes me as a guy who is a "Seahawk." Love watching him play and think (like Watkins), he's as safe as you can get as a WR prospect. Not to mention, he's Harvin insurance but can play outside and contribute in Special Teams. Seems like a really tough player who is extremely level-headed for a WR.
From a strictly "who helps the team the most" perspective, Greg Robinson, Jadeveon Clowney, or Aaron Donald would be the biggest no brainers.
Greg Robinson would be a slam dunk. We could start him as a rookie at RT, or flip him to LT if Okung got injured again. It would also allow them to start Bowie inside at LG. Of all the draft choices, he would be the biggest difference maker and would eventually replace Okung if they couldn't agree to a reduced salary.
LT Okung, Bailey LG Bowie, Carpenter C Unger, Jeanpierre RG Sweezy, Schilling RT Robinson, Hauptmann