Pass rusher/defensive tackle.
As much as the offensive line is upgradeable, they’re pretty solid and I wouldn’t say it’s impossible to win a Super Bowl with an offensive line entering it’s third-straight year with the same top six. Continuity can be underrated.
But the pass rush needs every bit of help it can get immediately. I’m not sure you get 19 or 20 consistently effective pass rush games out of Nink, Chandler, Buchanan, maybe Will Smith and Jake Bequette at DE and Wilfork and Kelly coming off injury, Sealver Siliga, Chris Jones, and unknown Armond Armstead at DT.
They seem to be pretty picky when it comes to pass rush-only guys, and really, pass rushing in the pros is a lot different than doing it in college where one or two moves done well are enough.
I’d be happy with either, but it would be a little more exciting if they took someone like Kony Ealy.