Safest bet at any given time is Adam, largely by seniority - he's one of my oldest characters, and among my oldest characters he's the one with the most development. I always go back to Adam. Over the years he's alternated between being an antagonist and a protagonist, but his actual personality and story haven't changed much. He's a biologist with little capacity for sentimentality, a grand total of one (1) non-professional relationship, and a whole lot of patience. I'd post a picture but I'd have to do some digging to find one, and the payoff would be my art style from 2011 or something, and I don't want to look at that. :,^)
Recently, though, the ones who've been holding my interest are Al and Nick from Powerless Gods - two characters who have yet to be introduced. Al's an interesting character to write and draw, especially with the particular difficulties of illustrating the way he emotes - he wears a shaped mask that puts facial expressions out of the question, and since his legs don't bend at the knee and various other body parts don't function normally his body language is kind of wonky.
Nick's just really different from the majority of my characters. He's friendly and trusting, and I don't get to write that a lot - plus some of his particular problems are really personal to me.