I haven't read much of One Piece but from what I did read.. I liked Chopper and the two doctors in that arc. Zoro and Luffy are also pretty awesome as well. Arlong is my favorite villain in the story so far.
Ok, this is really hard as there are so many great characters.
1. Law (Favorite good guy. He's an interesting character)
2. Kid (Awesome devil fruit powers and he's kinda badass)
3. Pell (A forgotten side character by many who was present in the Alabasta arc but I thought he was cool)