Much like [MENTION=996]Tony Vargas[/MENTION] I too would have a hard time even coming up with a top-10 list; not counting convention games and one-shots (which would add several dozen more) I've played-as-player probably 60-70 characters over the years never mind all the many party NPCs I've played as DM. And if I started telling their stories - even just those of my favourites - this post might never end.
But if I had to choose one it'd be - at least for today -
Appppil Pagey. Mechanically, going in she was my attempt to replicate a 1e Illusionist in 3e. Then the dice got going, and what came out was a bright, cheerful, bubble-headed Part-Elf farm girl with Wisdom 7 and generally poor other stats - who somehow had become a Wizard(Illusionist). Her odd name came from my using a random-letters table to generate it. (though how good is "Pagey" for a wizard! Sometimes I love dice...)
She was a throwaway. I fully expected her to die off in her first session, and already had the next character half-done.
But then something amazing happened: she didn't die in her first session. Or her second. Or anytime in that first adventure. Or the next. And by the time she finally did die we had revival effects available; and she lasted for 14 adventures over 7 real-world years of play. In fact, at the time her career ended she was the longest-serving member of the party! Three times voted "Most Entertaining Character" (and second two other times) in our annual all-game awards, she wore every emotion on her sleeve...she had rivals, friends, lovers, and (for lack of a better word) character. And bucketfuls of luck: sometimes when I look at what befalls my other characters I think all my player-luck got poured into Appppil; she had no business pulling off what she pulled off, nor surviving long enough to do it. Maybe that's why she's my favourite, or among them: I've always liked against-the-odds stories, and she was certainly one.
Woe betide you, however, should you ever be so foolish as to interrupt her while she was casting a spell. This was brought home all too clearly when - on the first few times it happened - she rolled natural 20 to [female dog]-slap the person responsible, to much laughter around the table.
And she broke the world. At least, that's how she saw it: the DM had set up an adventure such that if we were successful we would change how the world worked (mechanically, we'd jump from 3.0e to 3.5e on the fly) and Appppil triggered this while off exploring on her own by opening a particular door. I role-played this as driving her a bit mad; she became paranoid that everyone who had just lost abilities/spells/whatever due to this change she'd caused would be out to get her, and she was (almost certainly) going to suicide once we were back in town because of this.
She never got the chance. She was killed by her own party (she fireballed them once too often with her Rod of Wonder, they turned on her, she was dead within the round) and not revived.
Lan-"chaos for the win"-efan