The No. 1 interior defender in the NFL is Aaron Donald of the Los Angeles Rams.
Put simply, there is no player in the NFL who’s more disruptive on a down-to-down basis between the tackles than Donald. He was clearly a top-five talent in the 2014 draft out of Pitt, and yet he fell to the Rams at No. 13. Two years into his career, he has used his undersized frame to his advantage: At 6’ 1" and 285 pounds, he gains ferocious leverage at the snap by getting under the pads of blockers and using his upper-body strength to move them back. Add in his quickness and pursuit speed, and there are long stretches when he’s just about unblockable, even though he faces double-teams on a regular basis. Donald amassed 11 sacks in 2015 and already has 20 in his short career, but his impact is about more than that. He’s an incredible run defender, and he racks up pressures in bunches—last year he had 79 in total and 51 defensive stops according to Pro Football Focus. He’s the epicenter of the Rams’ front four, and he’ll stay in that role for a long time.