Mobile Suit Gundam Seed
There are quite a few mecha series that have a lot in common with Code Geass but Gundam Seed probably has the most, and it makes sense as it’s from the same production studio, Sunrise. Just like in Code Geass, Gundam Seed follows a young man named Kira Yamato who ends up being forcefully thrown into a conflict he wanted nothing to do with.
Gundam Seed features many of the same plot threads that you find in Code Geass, primarily of which is the relationship between Kira and his childhood friend Athrun Zala. The two end up fighting each other on opposite sides of the war, at first without knowing they’re even doing so, just like Lelouch and Suzaku. The story also weaves in surprises on characters being related, themes of terrorism and elitism, and more in common with Code Geass. Of course, on top of that there’s plenty of mecha action to give you your fill.