Haha, that's the sweetest question i've heard in a long time. You're wonderfull.
You can tell who is your favourite OP character the same way you can tell who is your favourite in any franchise.
Or what your favourite movie is, or your favourite kind of food. It's just.. who do you like, and who would you most want to spend more time with.
You just... know.. it's just, who do you enjoy a little bit more than the others, if any.
My personal favourite is Usopp.. lots of reason, he is a gadget builder and story teller, two character types I highly enjoy. Like his energy, like most of the jokes around him, like that he is a bit of a trickster.
But the biggest main reason is.. I relate to that guy, so much! In my head, he represents both the worst and the best in ordinary humans. He is so inspirational overcoming his own weakness's and fears, stands up for himself in spite of what-ever fears he is having, and defies his urges to run away, so he can stand up for himself and become the kind of person he wishes to be.
That to me, is so inspirational, and I always get so happy and giddy when Usopp turns around and do some-thing brave, just because.. I am rooting that much for him.
I want him to suceed and become brave, i'm ready with my little Usopp flag rooting for him when he is fighting.. so that makes it pretty obvious doesn't it?
