I think the jury is still out on this one.
Maybe the best way to pose the question is to ask for opinions, as the question is currently phrased here, but part of me thinks that there may actually be a psycho-scientific answer. One day there may be conclusive evidence (and enough exhaustive study) to put a percentage on the degree of behavior governed by nature vs. the degree of behavior governed by nurture...but that evidence is still in abatement.
My experience suggests that nature does play a large role (in suprising areas) in the formation of our eventual personalities. It may also be worthwhile pointing out that humans are constantly adapting and changing so that there may be a greater propensity for nature to rule at some ages and nurture at others and there may also be an extremely complex set of forces at work which will complicate efforts to come to a firm answer to this question.