Armin's two most powerful memories would probably be 1) the beginning of the battle of Trost, when he sees all his friends eaten by titans by titans, and the only reason he is not eaten himself is because Eren rescues him from the titan's mouth, sacrificing himself in the process, and 2) being literally strangled to death by leonardo da vinci (believing himself to be the mayor of harrogate and a cultist of hasteur) while believing himself to be a heretical prophet.

Open to anyone who wants to make their character suffer!