John Locke is not quite my favourite character in Lost. But of my favourite characters, he is the one who hangs around the most, so when my other favourite characters are either dead or haven't arrived yet, he does just fine. John's is a tragic tale that develops into one of distinct triumph, before falling back the other way again into one of the series greatest what-the-fuck endings. He is Lost's defining character, its working spirit, and he's also a badass bald dude.
Across the first few seasons we're shown that John has been told for a very long time that he was Special, despite being born to unloving teenage parents who shivved him into adoption and then conned him out of a kidney when he went looking for them. Every time he felt swayed by compassion, it was thrown back in his face, and his bitterness over what his parents did to him led to the breakup of his only successful relationship and to the events that led to him being paralysed before the series begins.
It's revealed in Season Five that his belief in his own specialness, and his belief in fate and destiny, is sorta justified by time travel. By time-travelling to a few years before he was born in "Jughead", John himself inspires not the circumstances of his birth, but the constant niggling presence of people telling him that he's different. It's a stable time loop - John is trapped by it, in fact. He is destined to become the leader of The Others for the sole reason that he goes back in time and tells Richard that he is.
A large aspect of Locke's character was his faith, and this is typically where I would back up and sprout secularisms at you for the next couple of paragraphs. But as we've seen, Locke's faith is justified by the predestination paradox that he lives in. His faith is often put against Jack's "reason", but I think that's rather interestingly wrong. As a rationalist I'd usually support Jack in this endeavor, but the observable strangeness on the island I think pushes Jack's refusal to admit to seeing the Monster or the Button's importance into Arbitrary Scepticism.
Locke was defined by something of a grand contradiction. His life before the island had been somewhat pathetic - a man defined by his bitterness, obsessiveness and anger. On the Island, however, Locke was something distinctly more powerful - a man who the others considered somewhat mad, but whose strength of character made him a figure as forceful as Jack. Locke was, of course, a tad delusional - he shouldn't have relied so much early on on vision quests and such like. But the thing about the Island is that accurate visions seem to come with amazing frequency.
His power and his inner feelings of inadequacy were eventually exploited by The Man In Black. His entire plan in Season Five is to bring the Candidates back to the Island in order to kill them and ensure that Jacob has no successors. Locke is key to this - the only reason that he leaves the Island and tries to bring everyone back is because MIB, in the form of Christian Shepherd, tells him to do so. It was a gullibility - an over-reliance on faith and visions in order to steer, the way, and a blindness towards those who may manipulate that.
John Locke may have relied a little too much on faith to guide him, but most of his visions and beliefs ended up being right anyway. He was a character that had made his way up to the position of the Others' leader after spending his life as a man powerless to change his fate. The four and a half seasons that he's truly around are much better off for his presence - mainly because of his unpredictability and his ability to make up for his tragic past through total badassery.