[Farael]

I'm goign to go against general consensus and say that Merry would've been chosen as a back-up bearer. We know that Sam was a good choice, but at the time, THEY had no way of knowing it. And given Sam's personality, and the fact that he seemed unable to make his own choices, would've precluded him.

Furthermore, even as a Ring Bearer, Sam fails on his quest. True, he gets lucky and ends up saving Frodo... but when he's returning, he's not doing it because he wants to save Frodo but rather because he cannot get himself to leave him behind. If Frodo had been truly dead, Sam would've probably failed.

Now, Aragorn, Gandalf and Legolas are rather unlikely to accept that burden. Legolas might've if all the others had asked him to, but Gimli at that time was not ready to befriend the elf, much less let him bear the ring. Conversely, Legolas would've opposed Gimli as a choice. Boromir would not go to Mordor, he said so himself.

That leaves Merry and Pippin. Pippin was barely a teenager by Hobbit standards, but Merry was a bit older and somewhat more settled down. Now, he was far from being as mature as Frodo, but closer than Pip. He could (reasonably) be expected to have the same innate resiliance that Frodo had, the question is whether he would have volunteered for the task.

Because i believe that, if no-one else had taken it upon themselves, Aragorn would've taken the Ring to Mordor as he was both strong in mind and body and he understood the reason why not using the Ring was important.

Of course, even then, he would've been too strong a match for Gollum in the end, so I'd say that the ideal Ring Bearer would've been Merry, but odds are that Aragorn would've been the one to take it upon himself as Merry would have not volunteered.

Either way, the quest would've been likely doomed without Frodo, as Boromir shows with his quote by Elrond.