Hands down, without a doubt in my mind, Beckett. If he went down, the rotation would have looked like this when Lester was on the DL.
Lackey, Wakefield, Miller, Weiland, Millwood.
I know Ells/A-gon/Pedroia are having monster seasons, but lose any of those guys, and the offense is still pretty damn good. Lose Beckett, and look at that rotation.