Receivers are the most likely to make an impact in their rookie season. I suspect the draft is where we pick up receiver(s).
Actually, running backs tend to have the highest "rookie ceiling".
Here's NE recent draft history for WR: (these are the guys that made the squad and actually started games) - All Hail FA!
Julian Edelman - 1st year - 37 REC, 359 YDS, 2 TD (*5th Season lead NE in Yds)
Aaron Dobson - 1st year - 37 REC, 519 YDS, 4 TD
Taylor Price - 1st year - 3 REC, 41 YDS, 0 TD
Brandon Tate - 1st year - 0-0-0 (He was the 3rd rounder the year they took Edelman in the 7th)
Matt Slater - 1st year - 0-0-0
Chad Jackson - 1st year - 13 REC, 152 YDS, 3 TD
Bethel Johnson - 1st year - 16 REC, 209 YDS, 2 TD
Deion Branch - 1st year - 43 REC, 489 YDS, 2 TD (*2nd & 4th Season lead NE in YDS)
David Givens - 1st year - 9 REC, 92 YDS, 1 TD (*3rd Season led NE in YDS)