[The Squatter of Amon Rûdh]

Actually, judging by the effect that the Palantír had on him it could well have been Pippin that cracked first. He's younger than the other characters and really doesn't understand the gravity of their situation. Eventually, of course, the Ring could have corrupted any of the Fellowship, using whatever weaknesses it could find. Boromir fell easily because in him the desire for a great weapon to use against Sauron was combined with a lack of self-knowledge and complete ignorance of his danger. Gandalf, Legolas and Aragorn were very much aware of this, and it would have taken a lot to make any of them forget the peril and take the Ring anyway. In the end, though, nobody who cared in any way about power or control could resist the Ring. Only Tom Bombadil seemed not to feel its temptation and the implication is that this is because he didn't want the only thing that it could offer: power over others.