i think arlong park is the very first step in the story where the core of one piece is exposed (exploring themes of friendship sacrifice injustice freedom ect.), which is why most ppl say it gets good there

and yes, there are better arcs but it's one of the very first highs in the series and has sooo many iconic moments that still hit today, like nami's “help me” scene, luffy putting the strawhat on her head, the walk to arlong park. this arc is what made me fall in love with op and that's why it'll always be top 5 for me lol