I have spent some time thinking about this. In responding, I am going to make the assumption that JT is a seasoned veteran and solid play from him is a given. The QB on any team is the most important position, so including him in my list just seems superficial and obvious. I will go in descending order of importance (1 being the most).
1. Shamire Devine - He is easily the player up front with the most long term potential. There is no opposing player who matches up against him physically. The biggest question in my mind is surrounding leadership and conditioning. If Shamire shows up in great shape with a clear and focused head, willing to be a leader, in every sense of the word, he will be THE guy a successful line can be built around. It is hard to put into words how much we need him.
2. Freddie Burden - We need him in top form. He makes the calls. He has the quickness to get to the second level and wreak havoc. He has the maturity and the experience to be a captain out there.
3. Marcus Marshall - We need him to hang on to the football. We need him to do just a little bit better getting the tough yards in the trash and a good bit better blocking. The rest is going to happen on its own. He is explosive and he will get huge yards with a little daylight.
4. Keshun Freeman - This is probably the first year he shows up physically ready to play his position at this level. Unfortunately for him he was rushed into service out of necessity, while a bit undersized for his skill set. He is a talented kid who is hard to block. From my distance, as a fan, he appears to have a lot of leadership skills. His character is very apparent. Having him blossom this year as a force on the edge, would be huge for the defense. If he can reach the level of Attaochu, let's say, in terms of pass rush, it would be a huge shot in the arm for the rest of the guys on the line. When you have to account for a player, in terms of blocking assignments, it means a lot.
5. AJ Gray - It has been a couple seasons since we had an elite playmaker (at any position) on the defense. DJ White and Adam Gotsis came close to this description, but not quite. Plus, AG was hurt or suspended for much of last season. IMHO, for us right now, AJG has the most potential to be that guy. When you have a guy like that at safety, it can really give you a lift.
Bottom line: all positions are important. I don't mean to slight anybody. If given a number 6, I would probably go with a WR stepping up. With all this said, if all of the questions in my numbers 1 through 5 were answered resoundingly to the positive, it is hard to imagine us not having a very good year.