Boromir- the first to become corrupted.

[hobbitlass]

Poor Boromir. He really had a lot of 'strikes' against him. He was a man to begin with, and as we all know are the easiest to corrupt.
He had a lust for power/position. Remember Faramir explainig to Frodo how Boromir couldn't understand why his father didn't take up the kingship, therefore he would be king next. Ad this was at an early age, too.
And as you pointed out, much worry and stress over his people in the fighting against Mordor.
Even though he obviously knew of the ring,he didn't seem to know of its corruptive powers and therefore didn't know to be wary and guard against the corruption.
So all these factors did make him the most likely candidate to 'fall' first.