G-d tells Avraham, "Walk before Me and be complete." (Bereshit 17:1) Rashi says that Avraham's completeness, his capacity to be TAMIM, was obtained by his observance of the Mitzvah of Brit Mila. As long as he had the orlah (foreskin) he had a physical imperfection. With its removal, Avraham now became complete.
The question is why Avraham needed the Mitzvah of MILA to become a TAMIM (complete). Noach was already called a TAMIM without the Mitzvah of Mila. What therefore distinguishes Noach from Avraham?
We find that that Noach's righteousness is the topic of a dispute in the Talmud, as quoted in Rashi Parshat Noach. Was he a Tzaddik only because he lived in a generation of wicked people? Had he lived during Avraham's time, he might not be viewed as being such a Tzaddik compared to Avraham. The flipside is, if Noach was able to maintain his virtue living among such evil people, surely had he lived in Avraham's period he would have been even a more outstanding Tzaddik.
Noach's righteousness is disputed, while Avraham's is not. Why not? What distinguishes Avraham's righteousness from that of Noach? Secondly, if Avraham's virtue is superior to Noach, why was it necessary for him to have a Brit Milah in order to be a TAMIM while Noach is considered a TAMIM even without Brit Mila?
The answer may lie in the origin of their righteousness. Noach was a Tzaddik, having been educated by his righteous grandfather Mesushelach. We can call Noach an FFB (Frum From Birth).
On the other hand, Avraham was born into a family of idol worshipers. The Rambam says that Avraham himself worshiped idols because that's what Terach, his father, taught him to do. Avraham achieved the truth about the One Living G-d on his own. He worked at it. He questioned his father's paganism, and when he received no satisfactory answers, he saw the fallacy of idol worship.
Avraham was the original Baal Teshuva. He was a self made Tzaddik. He made it on his own, unlike Noach who was born into the true faith and just coasted along. Thus, Noach was considered a Tzaddik Gamur (FFB) while Avraham was the quintessential Baal Teshuva.
The Rambam in Hilchot Teshuva rules that a Baal Teshuva is superior and greater than a Tzaddik Gamur (FFB). Therefore G-d chose Avraham to be the father of the Jewish People and not Noach. It was essential that Avraham's form of commitment, the unrelenting search for the truth, become an inherent characteristic of the Jewish psyche. It was necessary that those Jews who were born into unobservant homes and were taught pagan values should inherit Avraham's virtues and convictions.
The Mitzvah of Brit Mila signifies self-sacrifice. The shedding of our blood symbolizes our devotion and dedication to serve G-d. G-d gave the Mitzvah of Brit Mila to Avraham whose devotion to G-d was self-taught. He would bequeath to his children this ideal that one can achieve spiritual closeness to G-d even though his parents did not possess this conviction.
Thus, Avraham is superior to Noach, because Noach was FFB, but Avraham was the trail blazer of all BT (Bali Teshuva).