I'm going with Stallworth... no offense to anyone else on this list, but the lack of a deep game last year was debilitating to the team's success last year, and although no one agreed with me at the time, it had an adverse impact on many aspects of the offense, allowing the defense to move up towards the line of scrimmage, double teaming Watson, limiting yards after the catch, curtailing the running game and putting a lot of extra pressure and sacks on Brady.
That now changes with Stallworth & Moss - though give Stallworth the advantage seeing as his YPC and overall numbers were better than Moss's last year.
Conversly I think the defense did pretty well last year, as they always seem to, and though I don't minimize at all what AD means to this team, we'd still have a great D without him.
Welker will be an improvement over Caldwell (i.e a 10-12 ypc guy) - though he'll benefit from the fact that there's a deep threat whereas Caldwell never had that benefit last year and still hauled in 61 passes at 12 ypc - I hope Welker does at least that.
But I think the biggest need for the team is a deep game, and that's Stallworth and Moss, and Stallworth gets a slight advantage over Moss given last year's numbers