I'd say it's a 3 way tie between Tate, Sherman and Browner.
You can see improvement from Sherm and Browner throughout last year, but Sherman is a completely different player this year than last year. He's literally in a different league.
Browner also improved last year. He had better pass defensed and INT stats last year, but that is because QBs threw at him a lot. This year, he literally just erased guys. He Asumogha'd them. The WRs would be so far off of their routes or timing that QBs didn't even attempt to go that way, and when they did, Browner had tight coverage.
Tate's improvement was obvious.
The other 2 guys that improved most from start to finish of the year: Wilson and Wagner. Those guys just had a steady climb of progression that was amazing.