-( Story related, cannot be missed )-
#4 Dramatic Log: Summit War arc
This is the story arc of Luffy going to Impel Down to try and rescue his brother Ace, the Whitebeard Pirates fighting against the Marines, and in essence the whole of Marineford being a battleground among so many different people. If I remember correctly, the final fight is against Teech using both the Dark-Dark and Quake-Quake fruit abilities.