I think, logically, it would almost have to be Merry after stabbing the Witch-king behind the knee in the Battle of the Pelennor Fields. After that he is no longer essential to the rest of the story; even the Scouring of the Shire might conclude more-or-less the same way without him. Alternately, the same might be true of Sam if he sacrificed himself by throwing himself into the Crack of Doom taking Gollum with him.
Outside of the Hobbits, either Legolas or Gimli (or even both of them) could have fallen in the Battle of the Pelennor Fields or in front of the Black Gate.
On note of how important they are Boromir dies early on anyway . . .
Or Gandalf, because he comes back, but I would not risk his wrath.