This series is one of if not my favorite of all time. If I had to pick only one moment, it would have to be from ME1, when Sovreign is attacking the Citadel. More spefically when you have to go out into space and fight on the side of the ship. This is the most epic fight in the whole series for me. The fantastic muscial score is blasting the whole time, waves of Geth are pouring out from the ship, the battle between the Alliance and Sovreing is raging in the background, it just makes my heart race.