Here is my personal list of the best and worst. They are numbered in order of accuracy, censorship, horrible dubs, change of openings and changes of names. I will also include the name changes in the manga.
Dubs which info is practically nonexistant were not included.
No racism, patriotism or stereotipes were included in this. This is purely from a quality perspective.
1. The Japanese Dub:
Flawless and perfect. The best casting I've ever seen for any actors. Every single voice actor feels accurate and perfect to the manga, like they were BORN to do the role. They do help alongside the amazing orchestrated soundtrack to make certain scenes much better than the manga. Absolutely flawless.
Purely orgasmic:
https://youtu.be/Vc5DlwMZQ6A?t=1m29s
2. The Catalan Dub:
Really accurate to the original japanese one. Many voices sound similar to the originals (except some exceptions, I'm not exactly that much fond of Luffy). No censorship or bullshit like that. The openings are either japanese or dubbed well. Most names are kept intact except very few exceptions.
https://www.youtube.com/watch?v=eIzsoZCHLLY
3. The French Dub:
The voices are rather accurate to the original version and sounds rather good in general. There are some fuck ups in name dubs but not many, with most being literal translation. Also the intros are usually japanese.
https://youtu.be/cRyospEJr9k?t=25m16s
4. The Funimation Dub:
While they didn't censor anything and kept everything pretty faithful they suffer from one big problem: The dub is rather average. The big issue is that they try to make it sound like a cartoon so every single character speaks incredibly loud and like they were speaking to a dog. It makes it incredibly irritating. Plus some voices are really awkward, like Luffy sounding really really dumb. I know many like it but I do have my issues with it.
https://youtu.be/krGnypdGkbA?t=13m
5. The Russian Dub:
It's pretty good for the most part but it does suffer from some text translations. In general it sounds very decent and accurate.
https://www.youtube.com/watch?v=jErcPSWn8q8
6. The Spanish Dub:
While I cringed a lot with some of it's voices when I was a kid I have to admit the quality is overall rather good. Most voice actors sound very decent but some of them sound really off, like Luffy and Usopp... oh wait no, he's called USUFF in this version... But except that everything is pretty normal and the openings are dubbed properly. However it suffers from a lot of stupid name changes and there's even censorship and changes of names into spanish...
And Chopper's voice is absolute shit. Like holy shit.
https://www.youtube.com/watch?v=EtLi8oL1XDI
7. The German Dub:
While average for the most part there's a bit of censorship and way, waaaaaay too many names were altered. Not massive alters but holy shit how many names they changed. Many names are fuck ups on really basic errors.
https://youtu.be/KSFlsV3jcs0?t=6m12s
8. The Philipines Dub:
Extremely derpy sounding. Like really derpy.
https://youtu.be/u1jc5omapRE?t=5m6s
Plus wierd change in logo:
9. The Portuguese Dub:
The dub is really bad and they even used the 4Kids version. Since they weren't the ones to make it I won't count it as badly as the original.
https://youtu.be/BY6BMYB5TSc?t=4m
10. The Arabic Dub (Middle East countries):
There was a lot of censoring (heck, the skull in the logo was completely gone)
like removal of cleavages and violence but the worst has to be the awkward dub and hilarious opening.
https://www.youtube.com/watch?v=eCTTzec3Z7Y
11. The Odex (english) Dub (from Singapore):
OOOOOOH MY GOOOOOD
How can you do such bad dubs? Like holy shit this is bad. This must be some of the most cringeworthy unprofessional english dubs I ever heard.
Listen through the whole thing. It's worth it:
https://www.youtube.com/watch?v=7UJUiU3x0Vc
12. The South Korean Dub:
This version was heavily censored, with very stupid censoring. Not just covering women but also they invented the Sanji lollipop before 4Kids!
Zoro's swords are replaced for black sticks:
I guess insert Dildo Joke? ^
Censoring like a Pro ^
13. The Thai Dub:
Not only the dubbing sounds awkard as hell
https://www.youtube.com/watch?v=iJlFmyoNWK4
but the censorship must be some of the most hilarious censorship ever done.
Yeah, put a blurr effect over his blood, I'm sure nobody will notice.
14. The Italian Dub:
Not only did they change the title to "All'arrembaggio" (basically something the pirates screamed before raiding an enemy ship), not only did they make 3 horrible openings,
https://www.youtube.com/watch?v=f1PnYXCav5Y
https://www.youtube.com/watch?v=fIWgFXXHjJI
https://www.youtube.com/watch?v=6zkkp4vjp4I
"A game of thrones"? ^
not only did they censor it, not only the dub sucks ass but they changed so many names. Luffy is Rubber, Shanks is Shenk among many other stupid name changes.
The dub is bad. Like REALLY bad. Like REALLY REAAAAAALLY bad.
https://youtu.be/UY3YQl4Zd6Q?t=5m45s
Ironically enough it's one of the most up to date dubs.
15. The Greek Dub:
Oh my fucking god...
Intro lyrics that are completely different from the original,
https://www.youtube.com/watch?v=rCVWOTlB8Wg
the change of the title of the series to "NTPEHK" (Drake in greek), the intro sounds like it says nigga at some point, the derpish logo ever,
HORRID DUB, Luffy is changed to Drake, Nami is changed to Bonnie, Zoro is changed to Blackjack, Usopp is Gus, Sanji is Sunny, Chopper is Billy, Kuro is Black Pit, Arlong is Richie, Coby is Ross, Arlong is Richie, Mr. 9 is Hulio, Nojiko is Nora and Merry is MARIO.
One thing is changing names but the other is this shit. I can't believe this ever passed production.
16. The 4Kids Dub:
It's 4Kids for gods sake.
Their absurd censorship, satanic dub, profanatory change of music, removal of any violence or character development, the abolition of some of the most fundamental scenes in One Piece, change of opening and bastardization in the most flawless series of all times (in my humble opinion).
Go have a good laugh:
https://www.youtube.com/watch?v=MnU0czVRsHY
Honorable (shitty) mentions:
Malaysian dub calling the series "Rubber Boy"
Taiwanese dub calling the sereis "King of Navigation"
Also check this out to see every single fuck up in the names: http://onepiece.wikia.com/wiki/Name_Variants
Leave your favorite and most hated dubs below! ^^