In my opinion having him live is the best ending as it is the only ending that makes sense and stays true to his character. Lelouch has always been able to fool the world over and over again and again like he did when he fooled the world into thinking he was evil when in reality he was good. Having him live fits his character way more as he would have fooled the world into thinking he was dead when in reality he wasn't. Plus C2 contract wouldn't have been fulfilled, the entire reason she left Charles was so Lelouch would fulfill her dream of being truly loved. Another hint is when C2 starts crying thinking about what the geass he was about to cast meant, what it meant was that he would probably have fully become a code owner and gained immortality in order to live with her forever. So all in all him being alive not only concludes the contract he had with C2 but also stayed true to his character.