I'm going with RB and I'll give you my reason why.
Our 2003/2004 season we had Ricky Williams a great RB, we had a lousy O-line, no depth at WR and Jay Fiedler at QB, we almost made the playoffs.
Our 2004/2005 season we don't have Ricky Williams and we have basically 3rd string RB's and practice squad RB's as our starters, we still have a lousy O-line, more depth at WR due to Booker and Jay Fiedler and AJ Feeley at QB, we have the 2nd pick overall in the draft.
This year we actually got coaching on the offensive side that know what they are doing, much better than last year, we did make some slight changes to our O-line ex. McDougle. Our depth at WR is basically the same. We have Gus Ferotte and AJ Feeley at QB, imo Ferotte is actually an upgrade from Fiedler plus he knows the new playbook. I hate to do this but look at San Deigo, they passed on Vick aka Mexico and traded down and took LT. It shows that you can build your team around your RB to open up the passing game. I'm not saying Vick is any better or worse than LT but I think it shows that you can build your team around a RB or QB on the offensive side. Also look at some teams that have gone to the Super Bowl. Carolina had great RB's and a great D and a decent QB. Baltimore had a great RB, great D and an ok QB. We have a great D, and decent QB and with a power stud RB we have potential to be great.