I'm a 17-year-old English girl who has a tendancy to write OC's because of the time periods she bases her fics in. Granted this does mean that I actively set out to write them instead of adding them in where needed, however I've found that by doing this I get to plan them out better than if I simply put them in a fic willy-nilly.
Of course this does tend to leave my fiction unread but on the other hand it has given rise to some of my best (at least in my opinion) OC's; even though they're evil, aloof and somewhat nuts.
Oddly, I do not have a single favourite OC of my own, so that honour goes to the group of Grindelwald supporters in my fic 'Project Mutare'. Although out of that particular bunch I suppose Anchusa Aderis - an ex-linguist who betrayed the Ministy to go over to Lord Voldemort, nicking quite a number of important files along the way - could be said to be my favourite (points go to anyone who can get why her name is interesting considering her actions).
Anywho, the best OC ever? Gwendolyn Cross, most definitely simply for the way she manages to be both so delightfully original while from the outside looking, for all intents and purposes, like a Mary-Sue.