I can't limit it to just one guy...I'll try and limit it to three so as not to completely ruin the premise of the thread though.
Offense: WR seems like the logical choice here as nearly every other position on the offensive side of the ball has a starter firmly entrenched. The sentiment for Stanback is pretty strong amongst the fans, including myself, and even though id didn't particularly like what I've seen from the very limited video of the recent OTA's...Isiah Stanback gets the nod.
Honorable mention: While I'd like to see a young lineman step up and start pushing the incumbents, I'm going to have to give this to a rookie...and although I wasn't the biggest fan of the pick, Felix Jones is my guy.
Defense: Like on the offensive side of the ball, I'm going to have to lean towards our weakest spot...safety. I've been an outspoken fan of both Watkins and Brown since they've made their entrances on the team, but I've got to give the younger guy the nod by the slightest of margins. He's reportedly added some bulk to his frame (something Watkins may have a difficult time doing without losing speed with his particularly lanky frame), and is one of our fastest players. I love the idea of having safties lurking back there with the speed of cornerbacks, and as much as I've defended Williams in the past, I don't foresee him being around in 2009. We need to essentially have three or four coverage safeties on the roster rather than two. Courtney Brown.
Honorable mentions: Greg Ellis' recent cries for attention made this one a tough call. Anthony Spencer. Carpenter is right behind him.
Rookie: I'm expecting too much from our draft picks to really call them breakout players(yes, i do realize I chose Felix Jones above). I'm hoping Felix exceeds expectations. We dont' need a breakout year from a backup CB or TE. Same goes for a third round RB. I don't foresee our day tow LB or any undrafted DE's making an impact either. I'm well aware that the love for this particular person has the "realists" slamming on the brakes already, but I'm definitely on the Amendola bandwagon. Danny Amendola, and I don't care how much of a longshot it is...I like the way the kid plays.
Honorable mention: The idea of having three potential #1 CB's is too tempting...and I'd rather avoid the inevitable "Jenkins is a bust" threads. Mike Jenkins.