Aragorn: older, experienced, battle-scarred, grim, cares about the people under his command but not overly affectionate. But a really good guy.

My reasons for the two younger ones is: Pippin is youngest, sometimes with a great deal of youthful enthusiasm. Merry is a little older, a little more level-headed, and has more terrible things happen to him (getting shot by white supremacists=falling under the Black Breath).