Great question. This is a very interesting, and very old debate. I believe it was Augustine who solidified the now long-standing theological position that pride was the fundamental sin. However, I think there are a few other possible ways of looking at what the root cause of sin is. Pride is certainly one option, and I think Tolkien did a great job of allegorizing pride as fundamental in the opening chapter of the Silmarillion, for example, when Melkor, who apart from Eru has the greatest power and knowledge, breaks away from the song of the Ainur bringing discord into the world. That story just seems to illustrate pride as the fundamental sin in a very powerful way.
Still I think one could say that doubt is the fundamental sin, or, lack of faith. For example, if it was the case that God, being Who He is, had to some degree hide Himself from Adam and Eve so that they would even have the capacity to exercise their free will (for to see God directly would be equivalent to being overpowered by His glory, and not having real freedom of the will), then at the moment when Adam and Eve’s knowledge of God was limited (which perhaps is from the very start of their existence), there was fundamental lack of something right from the beginning in Adam and Eve’s experience, i.e. a full knowledge of God (and here I mean both cognitively and relationally).
That space of not knowing God fully, that space that God Himself allowed Adam and Eve, creates a doubt, a doubt that subsequently the serpent cleverly identifies and plays of off. Thus, in order to satisfy or relieve that doubt, Eve reaches for the fruit. As such, the first movement in Eve’s soul is not one of pride in the sense of wanting to be as great as God, but one of doubt and wanting to satisfy that doubt. Now, I think once the fruit is eaten and her, and then Adam’s, eyes are opened to the moral law of God, i.e. to the knowledge of good and evil, then the fullness of original sin develops, that being the desire to be good without relationship to God, and here we now have the coming into existence of pride, which then leads to self-exaltation or self-idolatry. So, while I think self-idolatry is connected deeply to the original sin, I think doubt is actually the first movement in the soul away from God.