Probably this guy! I've had him for four and a half years (which means he's not my oldest character, but still one of my older ones) and he originally came to me in a dream. I'd had no thoughts about making a character like Crowley before it happened. From the dream, I got ideas for his design (both human and dragon), his personality, parts of his backstory and his hatred for his younger brother. I woke up and instantly had to draw him so I'd remember what he looked like. While drawing him, the name 'Crowley' came into my head and I knew that was what I had to call him. Since then, he's been my main character (if I can say I have one at all) and my most recognisable character - people often say they like him, which is cool. :)
There is actually a separate thread for this question, but: are there any particular traits/characteristics that you tend to avoid when making characters?