Death Note
Death Note might not feature giant mecha or sci-fi elements, but thematically it probably has the most in common with Code Geass. The central crux of both series revolve around young men who are granted an unbelievable power, and use it to invoke their own sense of justice. In Death Note, Light Yagami is granted the power of a notebook belonging to the Shinigami Ryuk. By writing a name and the reason of death in the notebook, Light can bring anyone in the world’s life to an end. This power certainly has parallels Lelouch’s Geass that lets him control anyone he can make direct eye contact with.
Both Death Note and Code Geass put a huge emphasis on character, and the clashing ideologies of two opposing forces. In the case of Code Geass it’s Lelouch and Suzaku, and in Death Note it’s Light and the brilliant detective L. If you’re into Cod Ceass, Death Note is a thrilling choice that you might really enjoy.