-( Story related, cannot be missed )-
#3 Dramatic Log: Enies Lobby / Water 9 arc
This will wrap up with a fight in Water 9 as everyone gathers to board the Thousand Sunny. Usopp will be the last character to join up, for obvious reasons earlier in the arc, but once you've completed that episode, you should end up completing the arc itself. Three down, only four left.