(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.

[Doktor Howl]

Club Foot Jones in PF1e

Pathetic halfling with a club foot (-5 ft movement, cannot run), cook/torchbearer, pathetically grateful to have a job, always talking about saving up enough money to marry his girl (who he has a picture of in his locket), etc.  Looks and sounds like the woobie.

BUT.

He is in fact just a 2nd level expert, but he is also neutral evil and owns an amulet of nondetection with a permanent magic aura spell cast on it to make it seem non-magical (the locket).  He has maxed out bluff to including taking the skill focus (bluff) feat. 

And he aims to rob the party blind.

However, he is aware that vengeful PCs will chase him to the ends of the Earth, so they have to go.  So what he does is steal from each PC when they're for some reason not paying attention.  Then he puts the loot in another PC's gear.  If caught, he bluffs his way through.  His intent is that the PCs come to blows, then he picks the bodies clean.

If it looks like the PCs won't go after each other, he will wait until they're in town and then steal their shit and use bluff to shift blame to some no-name NPC at whatever inn they are staying at (he will in fact plant some of the lesser valuables on the NPC.)

If that doesn't work, he will then work on blackmailing the PCs through a 3rd party, who will afterwards suffer a tragic accident.

He will always have a plausible story.  Remember that he is a skill monkey with no magic other than his amulet.

If discovered, he will flee.  If captured, he will plead desperate poverty trying to feed his wife and 6 children, hoping the PCs forget about the "saving up to marry my girl" thing.