Who, what member, in your estimation, regarding active bands, is absolutely, irrefutably irreplaceable? No ifs, ands of buts, it's game over if this person were to leave for any reason (might be the front man, might be the guitarist ...), but there is simply no way, no how, that this person could ever be replaced. Even if *all* other members of the band are key, this person's absence would put the band in the tank.
What do you think? ETA: Then again, Queen survived, not that everyone has been satisfied with the replacement(s).
I would name Eddie Vedder, for starters; No Vedder, no Pearl Jam. Seems that way to me, but you may think there's a replacement out there in the blue.