Merry or Pippin

[Selmo]

I have always considered Merry to be the better, most mature and thoughtful of the two

In the early chapters, Merry is the planner. It's Merry who makes the house at Crickhollow available, it's Merry who arranges for Fatty Bolger to stay in the house to give the impression that Frodo stays there and it's Merry who organises transport for the party and provisions for the journey. He gives thought to the little things too, like having three baths ready so that Frodo, Sam and Pippin don't have to wait their turn to get cleaned up.
As shown by his comments after he and Pippin escape from Saruman's orcs, Merry spent time in Rivendell studying maps while Pippin just enjoyed himself.
After Merry and Pippin are separated, Pippin offers his sword to Denethor on impulse; Merry gives his fealty to Theoden only after some thought, because he has come to love the old King.
In the closing chapters, Pippin plays the impetuous Lieutenent to Merry's thoughtful General. Pippin rushes off to gather troops from Tookborough while Merry takes command at Bywater, planning his campaign with Farmer Cotton.

We should not be surprised that Merry shows the most maturity of the four Hobbits of the Fellowship. Sam, before the War, had little responsibility other than looking after the garden at Bag End. Frodo, with his inherited wealth, had nothing to do except study Elvish, write poetry and dream. Pippin was too young (under 33) to take on the burden of being heir to the Thain. Merry, however, was of age and would have been expected to take part in the running of Buckland. He was being trained in organisational and leadership skills in a way the others were not.
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