I don't know what my favorite phrase in One Piece is but I like what Luffy say's in this opening:
Luffy: I DON'T KNOW HOW TO USE A SWORD, SHARKFACE!!!
I DON'T KNOW HOW TO NAVIGATE EITHER!!!
I CAN'T COOK!!
AND I CAN'T TELL LIES!!
I KNOW I CAN'T LIVE WITHOUT HELP FROM A LOT OF PEOPLE!!!
Arlong: Bwahahahaha … What a clever boy you are to admit your own helplessness!!!
What a burden it must be to have such an idiot for a captain.
I don't know why they fight so desperately to save you.
YOU LACK THE SLIGHTEST SHRED OF DIGNITY OR ABILITY!
WHAT GIVES YOU THE RIGHT TO BE THE CAPTAIN OF A SHIP!!?
JUST WHAT CAN YOU DO!!?
Luffy: I can beat you.
May not be a phrase but this is among my favorite moments in One Piece :)