The part in chapter 15 where Lelouch takes the time to completely and systematically annihilate Suzaku's entire motivation for staying with Britannia, to the point that he even acknowledges how ridiculous Zero is.
Lelouch: "It hurts doesn't it, knowing that this is all the result of your decisions and ideals, knowing that everything you set out to do after your father's death has led to this time and this moment. Tell me, was this all part of your plan to change the world from within, allowing Britannia to do to the world exactly what it did to your country? Tell me, just how many have died because of Suzaku Kururugi's hypocrisy? How many innocents died in Japan and are dying now because you chose to join the ranks of the enemy in some naive belief that you could change everything from within, when time and again you were proven wrong? Why was it that hope came from an exile in a Halloween costume instead of the son of Japan's last Prime Minister, who so loved his people that he gave his own father and self-respect to keep them alive?"
Oh, and the best part to it all? SUZAKU ADMITS THAT LELOUCH IS RIGHT!
The battle of Pearl Harbor, specifically the duel between Lelouch and Suzaku, which showcases Lelouch's growth as a combatant in the last eight years. The duel is for the most part a stalemate (mainly because Lelouch's tactics and knowledge of Suzaku's fighting style is enough to counter his opponent's superior skill and battle experience), and would've likely stayed that way for quite sometime had Lelouch not be stunned by Suzaku's revelation of the true reason why he killed his father, causing Kallen to intervene at the last minute.