Eh, I don't really think that Franky would be the "most original"… And Vivi doesn't fit the qualification of "damsel in distress" either, in that she was the one saving people half the time... You didn't rank Brook either, even though he may not join. And you forgot Carue as well.
Alright, I guess I willl rank them now
Monkey D. Luffy - Luffy is a pretty simple character, but in a good way. He is not your typical hero, and I feel he doesn't fall into the "shounen stereotype". Sure he does in some qualities, like the eating, etc… but his normal behaviors generally always "nonsensical and laid back", which is complemented by his devil fruit power.
Roranora Zoro - I love his fights, and his personality is hillarious. From getting lost to his rivalry with Sanji. Emotion wise he's the direct opposite of Sanji. He has them just like Sanji, he just chooses to hide them. Anyway, Zoro has been becoming a bit stale as of late. Hopefully this will change back. And of course, Santoryu is a nice original style of fighting.
Nami - She was really cool from the beginning, through the Arlong arch. But then, after Ussop made he the clima tact, she just seemed to become your generic pissed off anime girl. Her fights just became fanservice, and she lost some of her initial "fun" attitude. It's not really the fact that she stopped using her weapon like a bo staff or anything, it's just that every fight she has had lately just turns into some long draw out game. But she did seem to prove her self again in Water 7.
Ussop - He is amazing. It seems that he has trouble expressing his feelings sometimes though, and just snaps at people. (and no, not just in Water 7) If you pay attention in the series, he does seriously it to many of the crew at one point or another. His fighting strategies are great, and now he's even better with his Sogeking "split personality". He really shined in the Arabasta fight against Mr. 4 /Miss Merry Christmas, and against Perona at Thriller Bark. I'm not sure where Oda is going with his character, but he is definitely developing into something.
Sanji - I like Sanji a lot. It's too bad that I always end up rooting for his opponents, because he generally dominates them. Even though he hasn't show much character development, he's still a fun character that wears his emotion out on his sleeves and shows them proudly. His fighting is always nicely stylized too, which is interesting. And he has been shown to be able to improvise, while in tight situations (like when fighting Wanze) Oda also gave a "secret agent" feel to his character. (which is complimented by his theme song) And I do with that he would don the "Mr. Prince" thing again.
Nefertari Vivi - Vivi probably experienced the most character development out of any other Straw Hat. From her entry as an antagonist (Miss Wednesday), to nervously becoming a Straw Hat, to becoming a happily full fledged member, so becoming emo and depressed, to downright angry, etc… I also liked how she seemed to do so much during her arch, especially in comparison to the other Straw Hats archs. She took the lead, and actually saved the team countless times. Her arch also had one of the most difficult villains. Not just a Shichibukai, but a Logia one at that. Her weapons were also pretty cool. I suppose that Peacock String Slashers would be a combination between a rope dart and a chain whip, which makes for an interesting fighting style. She also had some funny character quirks, like bitting her lip whenever she became nervous.
Carue - He was awesome. I especially loved his chase sequence with Bon Kurei. Carue also made an awesome team with Ussop in Little Garden, and held his own against Mr. 5 He's probably the faster runner on the team as well. He even managed to "fly" for a short time.
Chopper - Chopper is a good character. I like his traits and how he actually acts like a child, shows fear, and worships Ussop. My favorite episode with him would probably be his filler with Nico Robin. A few things about him confuse me though. Like why he reverts to brain point as a "base form". Why doesn't he originally revert to his actual reindeer form. Anyway, fighting wise he is probably one of my least favorite. But still, he is a very useful, and amusing member to the team.
Nico Robin - Now Nico Robin is a good character. She had a interesting childhood as well. I don't really like her power though. It doesn't require much skill. No matter how strong the opponent she can just snap their bones to defeat them. It's like she has a constant mod, but is the weakest player when it's turned off. In Enis Lobby she was basically the "damsel in distress", and did the least amount in her arch, in comparison to the others. Infact, I didn't really like her much at all until Enis Lobby ended. Now she seems to have loosened up a bit more, and has proved to be a great addition, especially at Thriller Bark. One of my other problems with Robin is how over rated she is. I've seen time and time again One Piece fans saying that they "got into One Piece" because of Nico Robin. It seems that much of her fanbase consists of the male versions of Sasuke fan-girls.
Franky - Great guy. Nice and original. To be honest, I preferred him as a villain. But still, he's a great asset and even hs a catch phrase. His character has limitless possibilities, as he is a cyborg, along with him being on par with Luffy. A fine asset to the Straw Hat crew.
Brook - Good so far. Interesting and original. Also the only pervert besides Sanji, which may prove to be interesting later if he actually joins.
Alright… so I'll now list my favorites in order
-Sanji** (switches back and forth with Zoro from time to time) **-Zoro