I am just going to post up this tier list I constructed for the Grand Adventure version of the game. It is based off partly win/loss ratios in specific match ups but it is supplemented with a lot of theory-fighter bullshit. I posted earlier versions, and this one, in the shitty fighting game tier list thread. This post is a copy paste of the later one. If anyone wants more information or sees a problem in the list then I will be happy to have a discussion about it.
A tier:
Bon Clay (Bentham), Robin, Kuina, Crocodile, Kuro, Mihawk, Eneru
B tier:
Shanks, Smoker, Buggy, Zoro, Don Krieg, Usopp, Luffy
C tier:
Ohm, Zeff, Ace, Chopper, Wapol
D tier:
Arlong, Nami, Sanji, Vivi
F tier:
Tashigi
Character added to list:
Ohm enters the list after some more extensive playtime. He is extremely annoying and has a specific glitch associated with him that makes some argue for him to be banned, or at least use of that glitch, but for now I think he deserves a place on this. His placement is a bit hard to make since there is so much debate around him in my circle of players. I am throwing him at top of C tier because of his ability to force people to either burst or enter accel-heat mode in order to beat even approach him. He will be playable, glitch and all, at a tournament I will likely be holding soon. I will try to record all the matches in the tournament. Hopefully someone will play Ohm for some of them.
Characters that moved up:
Bon Clay moves up to #1. His crazy karate is extremely strong and can be more-or-less an infinite if the opponent has no special bar. There is even a player in my group that thinks he should be banned. I don’t think Bon Clay nearly as overpowered as that player does. I would actually say he is tied for first with Robin but I will have to get more match data so that I can make another match up chart to verify that.
Crocodile can control a lot of space. His ability to limit his opponent’s options are insane. If his opponent makes a mistake in their position in relation to Croc then Croc is going to have his way with them and drain them of their life. He is a bit hard to play which is why I thought he had worse match ups than he actually did. I used to think just Mr. 1 was his only viable assist but now I understand that Ms. New Year’s Eve is better for some match ups.
Eneru has probably moved around the most from revision to revision but he is back in A tier. A few more tricks that I didn’t even consider have been being employed by some friends of mine. These tricks make him hard counter a few character and increase his standing in match ups across the board. He is hard to approach when he hides in his assist, and if he is able to enter accel-heat mode he can throw out unblockable projectiles and other ranged moves. It is still possible to give him a decent amount of punishment for this but if you make a mistake you are likely to lose more than half your health.
Buggy controls an INSANE amount of space but often only does a small amount of damage. He has some strong combos and his pressure game can’t be stopped by a single burst. His higher placement is because it has been shown that his match ups are better than I originally had thought.
Characters that moved down:
Chopper moves down. He isn’t nearly as good against Eneru as I thought. Some of his other match ups I thought were favorable for him were not. Simple as that.
Usopp has a couple of his match ups have been shown to be be less favorable than originally thought. I wouldn’t be surprised if I raise him again on later revisions because he is an extremely difficult character to play and has a lot of strange tricks in his arsenal. However, from current theory and win/loss data, he has to go down for now.
Arlong is one of my favorite characters in the game because of how fun and flashy he can be. He hits like a truck and controls a decent amount of space. He may have a lot of safe and high range moves but his sluggish nature makes him have quite a few hard counters. I am working hard to find new tools to use against those hard counters so I am hoping I will be able to put him higher again in the future. However, it seems that as players continue to understand the game and the match ups that Arlong’s weaknesses are going to make him fall even further. 
Sanji is very flashy and brings a ton of pain on the people who are unfortunate enough to get touched by his glorious legs. This doesn’t help him when playing to win though. He doesn’t control much space and he has a hard time getting close enough to most characters to be able to apply pressure.
Other notes:
Kuro didn’t move from his position this time but he will probably move in some way next time. I understand his strength and weaknesses better since he has gotten a lot more playtime. Some of his match up scores are likely to go up, while others will go down. I have no idea where he will be in the revision.
Shanks scares me. I recently got another player to pick this game up and he has chose Shanks to be his main. He has a better understanding about which assists to use in which match ups than I ever did. I have seen Shanks do some pretty insane things. His playstyle changes dramatically depending on match ups so there is a high likelihood that the match up data I gathered on him is incorrect. After I get more time playing with everyone, specifically after the tournament, I will be able to give him proper placement. I am guessing that he will probably move up into A tier.
http://i362.photobucket.com/albums/oo64/rizuky21/One%20Piece/Nami.gif
Nami. I don’t have much data on her so don’t take her positioning too seriously. If someone knows about this game and plays Nami I would really love to get some information about her and her match ups. I feel like she has potential but I have been unable to figure out how to use the tools that she has to get around a lot of the tools that other characters have.
http://i362.photobucket.com/albums/oo64/rizuky21/One%20Piece/vivi.gif
Vivi is another character I simply don’t feel like I understand. She doesn’t get played enough for me to give her proper match up lists. I know she has a touch of death on Kuina, and likely a few other characters, however that alone is not enough to make me put her any higher. I hope to learn more about her in the future.
Luffy is probably going to be moving around next time. There is a Luffy player that has played with the people I most commonly play against and they tell me he is a formidable opponent. I am hearing some amazing things but have yet to see them myself, he is currently is on vacation out of the country, so I am hesitating to move Luffy’s position. I think Luffy is going to move up next revision based on what my friends have been saying about Luffy’s match ups.
Robin makes people unhappy.