http://www.youtube.com/watch?v=2dvJGePsqmU is definitely my favorite track in OP. Every time I heard it I get hyped. And it's usually followed by some pretty epic speech or prediction.
"Everything they're doing is pretty interesting. A sane person would never do anything against this island. Behold Chimmeny, Whether they die or survive in the end, striving to achieve what nobody in the past hundred years was able to do, this incident will astound everyone in the world. Once this battle is over, that Straw Hat brat's name will roar throughout the whole world!"
Luffy: *Deep breathe* ROBBIIIIIIIIIIIIIIN! I CAME TO BRING YOU BACK!!!
or
Mihawk: "I'm sorry, Red Hair. I will not hold back my power."
Luffy: "That's....Hawk Eye!"
Mihawk: "Now, I wonder if fate...will end the life of the child of the next generation here. Or if it will save him from this Black Sword!"