I read Geass fanfics over and over, so I can safely say which are my favorites, which I would recommend to just about anyone:
Code Geass Megiddo by Wing Zero Alpha: A wonderful portrayal of if Lelouch was kept in Britannia after R1, and follows Kallen and the Black Knights five years later, attempting to restore their vanished leader. Numerous good O Cs
, and a high quality take on Lelouch's brilliance without stepping into Gary Sue territory. Be warned, not for Suzaku fans.
Dauntless by Allora Gale: Lelouch is returned to Britannia after being seen by Clovis in the Tokyo settlement, and his navigation of the political climate as he attempts so survive his siblings and his father, while concealing that Nunnally lives. Updates quite quickly, good O Cs
, and a wonderful political game.
Lelouch of Britannia by Cal Reflector: Need I say more?
The Sum of Our Choices by Juubi-K: The Black Rebellion succeeds, leaving Zero the statesmen the task of keep Britannia of his shores by taking the whole show worldwide. Wonderful O Cs
, brilliant storytelling. May or may not be Dead, but worth the read regardless.
On the funny side, Living Contradictory
by erickmenk never fails to make me laugh, as the whole of the stadium listens in on Euphie and Lelouch's discussion prior to the Euphinator incident.
As far as Fix Fics, For Want of a Nail by Mith Luin
isn't bad at all, but it's kind of short and might be dead. Averts Euphinator by Lelouch agreeing to go along and join the Special Zone.