The opening of the S1 24-25 special, "Hitomi no Tsubasa", is quite possibly the most epic song in the series.
"Picaresque". This is when they broke Toudou out from his prison. With THIS music blaring loudly in the background. It really can't get any more badass than that. Also worth mentioning is "Callin'" by the same artist, which plays in Stage 20 when Suzaku and Lelouch as Zero work together to stop the Chinese invasion of Kyushu. Check the lyrics for these two songs and you'll notice that "Picaresque" can double as an Image Song for Lelouch while "Callin'" could be one for Suzaku.
"WORLD END" by FLOW, the 2nd opening for R2 and the last of the series, while not quite as good as "Hitomi no Tsubasa", deserves 2nd place. Also, R2's 1st Opening, "O2" by ORANGE RANGE and "COLORS" also by FLOW the very first opening of the series... Jibun Wo~!
4) And, of course, Lelouch's last words before he dies.
Check Mate. Lelouch and Suzaku breaking into the Damocles is totally awesome! Ironically this also plays when Anya pilots the Mordred to find Lelouch during the episode where everyone was chasing him.
Love is Justice. You can't get much more badass than firing a nuke, killing a few thousand soldiers, then proceeding for some equally badass monologue. They play it in the very last minutes of Turn 23, when the first FLEIJA is fired from the Damocles against Emperor Lelouch!
"COLOURS" is one of the most memorable as far as anime theme songs go.
Alternative Title(s):Code Geass Lelouch Of The Rebellion