I'd have to go to the beginning to remind myself of my favorite OP moments...
I liked the Luffy vs. Crocodile fight (When Luffy punches him multiple times and sends him flying
) ..that's 1.
2. The time when Luffy and Zoro get beat up in Mock Town.
3. Luffy vs. Foxy ...Luffy looked epic.
4. When Luffy calls out to Robin, while stading on top of the courthouse. ..And then when Robin screams out that she wants to live.
5. Sogeking shooting the WG flag.
6. When everyone around the world finds out about the STs taking out Enies Lobby.
7. The whole entire Thriller Bark arc (Zoro's epic moment of being the right hand man).
8. When all the supernovas were introduced.
9. Luffy punching that tenryūbito.
10. Luffy taming Sonia, Mari, and Hammock.
11. The return of Buggy the king.
..err, the clown. Buggy the clown.
Weird how I have no favorite moments before the Straw Hats cross the Grand Line. ...anyway, that's all I could think of.