My favourite is Luffy for his pure simplicity and likeability. I don't get why his personality and design are so obsessive but they just are, it's got something to do with the voice. Sanji is also enjoyable.
My least favourite is Don Krieg, man he sucked. Huge big talk and then he get's knocked down by one punch from Luffy.
That said, I AM just finishing the Arlong arc...so yknow, early days.