In between thinking great thoughts and other rabbi stuff, I post here.
Since the question is limited the Ten Commandments I would suggest that the First Commandment, "Love the Lord thy G-d for I am the Lord thy G-d who brought out of the land of Egypt." is the most the important because everything else flows from this. Rashi stated that a Mt. Sinair when the Lord spoke to the people of Israel gathered around the mountain after the he pronounced the first two Commandments the people fled for fear of dying after hearing G-d's voice and Moshe, rebbenu, convinced to stay but no look up, and so the first two Commandments were delivered directly by G-d to Israel, and the other 8 via Moshe, rebbenu. So would make the case for supporting the First Commandment as the important of the ten. Rashi, also stated that "love thy neighbor as you would like to be loved, as the rest is commentary." This on its face seems to make this the most mitzvah of all, but is it? Did Rashi intend to minimize the First Commandment? Then Ramban, stated that "Love thy Stranger" is the most important of 613 commandments because it's repeated 36 times in the Torah and many scholars hold this to be the important of the mitzvot. However, since this we're limiting ourselves to the first 10 I believe the First Commandment to be the most important.
Jason - great to hear from you and hope you're well. Thank you for your thoughtful response. Best, Aaron Benson