It's a tie between Zoro and Sanji.
I like them both very much but I can't actually say which one I like more. I love it how they both are so freaking cool but can also act like a total fools. And they can do this without being ridiculous. I have never lost my respect for them despite the fact how silly they both can be. Same thing with Luffy.
I also like their fightning techniques. Katanas and kicks . Zoro's fights are great but Sanji's are more original and more crazy which is only a good thing and that's why I enjoy more about Sanji's fights.
Sometimes they tend to use too much this "before" and "after" technique with Zoro (in anime).
Zoro is better looking and he has more interesting dreams but Sanji offers more variability because of his personality but then again I really love Zoro's way of acting: his overall laziness an all.
Aw DAMN IT! I don't know. My opinion kind of changes a lot.
Then there's a lot of other characters I like (These are not in order):
Rob Lucci: Cool and charismatic. A perfect villain. I love his design. Especially Hattori which is that one touch that makes Lucci so interesting.
Kaku: I still don't know how Oda managed to create a guy who has a nose and eyes like that but is still so freaking cool. Very original looking character.
Mihawk: Stylish and interesting. I would really like to know more about him.
Smoker: Really charismatic. And he's not only some stupid and boring Marine. This guy actually has brains.
Ace: Funny and cool.
Shanks: I don't really know why I like him so much. Maybe it is his cheerful personality. I'm also very interested about his past and all.
Buggy: This guy is freaking funny and original looking pirate. I also like it how he isn't like most villains. He actually changed in the series. I'm really looking forward for seeing him again.
Luffy: He doesn't actually interest me as much as many other characters but I still like him very much.
Umm… There are too many good characters in One Piece.