Since the question is "best development", I am going to assume it does not necessarily have to be a drafted player?
I'm only in the second season so barely have developed my rookies yet D:
My best development is probably Ricky Stanzi (QB) with the Chiefs after about 2 seasons. He got the chance since Cassel was too old, too expensive and not much better really (and I was unable to aquire Mallet or Osweiler, since it's an online franchise). Since being given the chance he has developed from a scrub to a hero! 92 THP, 90 in all AC's, about 80 throw on the run and PA. My plan with the team was to either trade for one of the two mentioned QBs, or end up drafting one after the first season. But Stanzi developed so well that he has turned into my franchise QB
Another player that I have had satisfaction in developing is DT Dontari Poe. He has some amazing attributes, but his awareness and play recognizition is super low (40's). Which means that he basically never was able to recognize the run plays up the middle and make a tackle. Now, after about two seasons of development, I have finally gotten him to a point where he sheds his tackle and takes down the runner for 1-3 yards gains, which before often was about 6-8 yards due to the tackle having to be made by a safety
Lastly is OLB Justin Houston. A power pass rusher. I have mainly focused on developing his speed, due to wanting to try out to create a combination of power moves and speed. This has turned out to be a deadly combination as he is currently the sack leader, and the top candidate for defensive player of the year. The best thing about him is that he still has so much room for development