-( Story related, cannot be missed )-
#5 Dramatic Log: New World Arc
This will wrap up the Dressrosa Arc and introduce you to Cavendish and Bartolomeo as fans of Luffy. While he may not take their sake cup of friendship as they wanted him to lead their fleet, he does instill a sense of innocence back into them for what he desires of the future, when he finally becomes the King of the Pirates. After this, there are two other arcs, so be prepared for some difficulty.