With a nod to http://www.49erswebzone.com/forum/thread.php?num=133613 regarding the most important position, I think an even better appraisal can be made by considering the most important grouping (breakdown courtesy of MM in his 53-man reviews) of players.
I vote OL. Without them, the offense goes nowhere. While I reckon the defense to be important, they are reactive...they don't initiate the play. That's why I rate the offense more important, and the OL the most important part of the whole process.