Steins Gate follows the eccentric and melodramatic Okabe Rintaro, who often calls himself "Hououin Kyouma" - an enthusiastic scientist who runs a lab called the Future Gadget Laboratory with his childhood friend Mayuri and his hacker friend Daru. With the accidental creation of a time leap machine and an encounter with the genius scientist Makise Kurisu, Okabe's world turns upside down as he clashes with a top secret evil organisation called SERN. The anime dapples with time travel and multiple realities logically while managing to engage the audience with the intricacies and consequences of such a world. Although the show requires some patience and investment for it to show the brilliance it's capable of - it still manages to segue the goofy tone of the show into something grounded and dark. Steins Gate manages to keep your brain working as you try to fit the pieces together in order to understand the gravity of the situation.
Steins Gate is a psychological sci-fi anime that reveals the horrors of time travel and taps into the emotional side of you. The character of Okabe Rintaro and his growth from a delusional maddo scientist to one that has the weight of the world's responsibility is an interesting watch. Okabe Rintaro's mental state of mind manages to project itself onto the audience who in turn find themselves hooked and desperate to find out how things would be put to a close. Another example of this projection is with the popularity of Makise Kurisu, who we see as a tether to sanity in an insane world, just like how Okabe sees her as she supports him in the ordeal. Steins Gate, in both its seasons, manages to pack some unpredictable plot twists without being overdramatic. Season one's ending was quite beautiful and provides satisfaction yet season two's ending, personally, was a little underwhelming despite the season being solid.
Code Geass, on the other hand, is a psychological mecha anime that begins with an exciting set of events that changes the life of Lelouch Lamperouge while giving it meaning. Code Geass is set in a Japan that is annexed by Brittania and stripped of its identity with the name "Area 11" where its people are treated as second class citizens. A young Lelouch Lamperouge determines to exterminate this empire and soon within years to come, Lelouch encounters a mysterious girl that gives him the power of the "Geass". Geass gives Lelouch the power to command an individual once. With this power, Lelouch, a genius Brittanian, leads a double life under the name of Zero, who leads a radical resistance against the Britannian Empire. Code Geass' plot plays with strategy, action, politics, love and can be considered substantial for a well rounded experience.
Our protagonist, Lelouch, is a man of gray morals. His motivation behind bringing peace to Japan is to protect his sister yet his actions reek of violent means. His genius strategies keep the audience hooked and his complicated personality makes him a favourite. The balance between Lelouch's hard and clever personality and the emotional side of him makes this human more appealing. Lelouch enjoys the power yet he's still conscious of his motivations and his loved ones. The tonal shift is more frequent in the anime but it is not abrupt. The anime tends to focus on Lelouch's school life and mellow personality and his second life which manages to make up for some comedic scenes. The show is a gradual nonetheless exciting journey to its amazing ending at the end of season 2. The overwhelming emotions one goes through at the end is unforgettable.
Although Steins Gate is a brilliant anime and it is one of the best I've seen, it tends to get confusing too whereas Code Geass manages to be a brainy show about strategic fights without making it too difficult. Considering the ending of Code Geass, its whole journey seems to be a beautiful lead up to it. Although Okabe and Makise are wonderful characters, the sole intensity and complexity of Lelouch overpowers them. As an overall experience, I'd say Code Geass gets the vote.