With the way the roster is constructed at this moment I would have to say Rondo. While I do not doubt that loosing Pierce/KG would be huge, loosing Rondo would be the biggest loss. We have no one with experience behind him. Hudson has looked good in garbage time, but that is garbage time, not managing those personalities 30+ min a night.
I think a combination of players could pick up the slack if PP or KG went down, but there is no one that could do the same if Rondo went down.
But knowing that we could probably pick up a serviceable pg somewhere down the road it would have to be PP. He brings too much to the table: leadership, scoring, defense, match ups, post play, rebounding, and maturity.
Lets just all knock on wood and pray this doesn't happen.