In terms of most indispensable - it's gotta be Rodgers, Woodson, Matthews, Collins, Finley, Jennings. None of them have capable backups. Last year, I would have put Clifton on the list, but with Bulaga in the fold, we may be able to handle an injury at LT.
Another way scouts do it is to list blue chip players - players that are among the best at their positions. With this question, I would add Sitton.
Packer Blue Chippers
Rodgers
Woodson
Collins
Matthews
Jennings
Finley
Sitton