Eren, a character born in chaos, is chaos and brought chaos. Everything about him screams greatness and good writing, there are no other characters which come close to his characters.
Some of his qualities,

1. From Hero to Anti-Hero to: He starts as a hot-headed MC seeking freedom and revenge, like a typical shonen MC. But as the story goes on, he evolves into a morally gray figure, questioning the very nature of freedom and sacrifice. By the final arc, he's completely unpredictable, making him one of the most dynamic characters ever.
2. Philosophical Depth: Eren's ideology isn't just about winning battles; it's about questioning fate, free will, and the cycle of hatred. His journey forces the audience to think about whether the ends justify the means. His famous line, "If someone won't take their freedom, someone else will just take it from them." shows how much his worldview changes.
3. Emotional Impact: His decisions carry massive emotional weight, not just for himself but for every other character in the story. Whether you agree with him or not, you can’t ignore the way he influences everyone around him.
4. Iconic Moments: From the Declaration of War speech to his final conversation with Armin, Eren has some of the most memorable scenes in anime. His presence dominates every scene he’s in.