The Amusement Park Arc is the very first arc of the Sakamoto Days manga and anime series. It covers the introduction of most of the core characters, the show's premise, and their first "big adventure", so to speak. Apart from the plot though, the Amusement Park Arc does a good job of establishing the feel of the series. Sakamoto Days is a comedy action show that trivializes some really dark things, mainly involving hitmen and assassinations. Another anime in the same vein as this would be Spy x Family, so if you liked the Forger family's misadventures, there's a good chance you'd like Sakamoto Days.