(For GMs) favorite PCs and their stories.

[Chucklemaster]

I'll start.
I was running a 4e campaign with some friends, most of which hadn't played d&d before, and my immediate favorite among the PCs they created was Lariss, a warlock who made her demonic pact when she was a teenager as an act of rebellion. problem is, the demon stuck around past her teenage years(blood pacts are tricky that way), and she tries to avoid casting spells these days, if she can avoid it, so she doesn't have to have too many awkward conversations with her ex-oathmate.

[Demolition Squid]

Running a Cthulhutech/Evangelion mashup campaign at university, one of the players always - always - played a samurai expy, no matter the game. We had western samurai, fantasy samurai, superhero samurai... this time I had a little chat with him before we started to set things up.

Over the course of the campaign the other characters - and players - became convinced he was just insane. He kept talking about his 'Master' and going to an old dojo, but the other PCs never saw this Master and when they went to the dojo it had been closed down for years. As a pilot he was terrible, but the payoff happened in the final session when the PCs needed to get to their giant robots through a small force of special forces troops- not exactly fair when you're a bunch of teenagers.

It turned out that the nerdy Japanese teenage samurai wannabe was a samurai wannabe because Nodens, God of Dreaming and Inspiration, had a direct link with him, blurring the line between fantasy and reality. When shit hit the fan and the world was going to end, Nodens revealed himself, gave his disciple a kickass katana, and let the kid go to town.

The reveal was totally worth it for the look on the other player's faces, and the player of that character really loved having an excuse to play a self-parody.