Biggest impact - that's a hard one because there are possibly two answers, one for the regular season and one for the playoffs (which will ultimately determine the season).
For regular season the biggest impact would have to be a starter by mid-season at the very latest, and so I would go with either McCourty or Gronkowski. While I think McCourty will do well, I suspect that the whole D will perform better and it will be debatable whether it is his influence, Spikes, McKenzie, Mayo, Chung, Butler, or even Crable or Cunningham. So I will go with the Gronk, because he will have an impact on our pass and rush offense that has been missing for the last 3 years.
For the playoffs I am mostly convinced that defense wins championships and so I am going with the person who will make the biggest plays in the playoffs, and that person is Spikes.
I will also try to answer the thread title, which asks for the biggest surprise which is very different from biggest impact. I am expecting the biggest surprise to be someone who makes the field more often than not and is in the 7th round to UDFA group - so here I will choose between one of my Binkies, Ted Larson, and Deaderick. Of the two Larson has the harder task in replacing Koppen, but I suspect he may surprise by subbing on certain downs. But for the money I have to go with Deaderick as his motor seems to be more in line with breaking expectations.