1. Captain Morgan Arc - 8/10. It shows the origins of Luffy and Zoro quite well, and effectively establishes Luffy's personality. Morgan may not have been the best villain, but he was a good start.
2. Buggy Arc - 6/10. Nothing really special, other than Buggy's antics and Nami's debut. Too few characters, and the Shanks connection seems out of place.
3. Captain Kuro Arc - 7/10. Slightly better than Buggy, although the pace is off. Fortunately, the annoying Veggie Pirates are compensated for by giving all the main characters fairly equal footing (in terms of screen-time).
4. Baratie Arc - 9/10. An unforgettable cast of characters, especially Gin, Krieg, and Zeff. Climactic battles combined with deep emotions. An ideological clash between Luffy's and Don Krieg's respective versions of "strength."
5. Arlong Arc - 10/10. Touching moments such as Nami's acceptance of Luffy's help, her reasons for doing everything, and so on. The first arc with "a fight for everybody," so to speak. Arlong, who radiates fanboy radiation. Sadly, though, Johnny and Yosaku never did anything.
6. Loguetown Arc - 8/10. Far too short, but a heaping helping of foreshadowing, character introduction, and plot development makes up for it. Dragon has become perhaps the most debated character in the One Piece universe. Buggy's triumphant return succeeds where so many "villain comebacks" have failed, and Alvida's connection is a surprising twist.
7. Reverse Mountain - 3/10. Nothing memorable here. Luffy's promise to Laboon is the only event keeping this arc from a flat zero.
8. Whiskey Peak Arc - 10/10. Surprised? I am, especially considering half the characters didn't do anything. This arc isn't much for plot at first, but Zoro keeps things interesting. The arrival of the #5 pair, the revelation of "Miss Wednesday's" true nature, and Luffy's fight with Zoro keep Whiskey Peak from being just another pitstop.
9. Little Garden Arc - 6/10. With all its long-winded rhetoric about honourable battle, this arc falls flat on its face. The villains are either recycled or outright uninteresting, and the message is unclear. One thing's for sure, though: Sanji, Luffy, Carue and Usopp manage to shine through the murk.
10. Drum Island Arc - 5/10. Giving such a low rating may get me labeled as an infidel, but hear me out. The supporting characters' backgrounds were lackluster and uninspired. Wapol was completely underdone, and Usopp and Vivi didn't do a damn thing! One gets the feeling that Oda spent more time on Chopper's backstory than he did on the arc itself.
11. Alabasta Arc - 9.5/10. Luffy's brother revealed! Smoker reappears! Zoro cuts steel! Every character gets a fight! The Clima Tact unveiled! A plan for every contingency! War! Strife! Conflict! Egyptian overtones! Poneglyph! Pluton! The most breathtaking conclusion since Arlong Park! Crocodile, Nico Robin, Bon Clay, Smoker, Pell, and the Straw Hats are the grand balloons struggling valiantly to lift this otherwise stellar story arc above the rest; Vivi and Kohza are the clumsy, leaden anchors weighing it down.
12. Jaya - 6.5/10. One word: ANTICLIMACTIC.
13. Skypiea - 8/10. The bizarre new world of the sky and its fantastic trappings (dials, wannabe gods, flying ships) are incredible. Unfortunately, an overly complex storyline bogs it all down into an incomprehensible mess. Eneru easily attains "Big Baddie" status right alongside Crocodile, but this arc still tastes like filler.
14. Davy Back Fight - 10/10. Am I the only member of this forum who doesn't despise the Davy Back Fight? A pirate competition with some laid-back fun quickly escalates into a struggle for nakama. Whiz-bang fun! {<–cheesiest phrase ever} The race is a showcase of skills, Groggy Ring is like football... with death, and Foxy manages to strike the perfect balance between comical and evil (he also possesses one of the most unusua abilities--it's right up there with the Doa Doa no Mi). Just when you think it's all over: BOOM! Admiral Aokiji, bitch! As Aldrich would surely say. ZOMG FORESHADOWING!!!1
15. Water 7 - 10/10. An emotional roller coaster that would surely receive two thumbs way, way up from Ebert & Roeper if they paid any attention whatsoever to One Piece. Franky epitomizes the term "character"… and if you don't like Kokoro, then you deserve a mermaid kick to the schnutz. The Galley-La Foremen and the Puffing Tom serve as more than just eye candy, too. The death of a ship. Two crewmembers lost almost simultaneously (three if you include the Going Merry). The unveiling of CP9 shocks and astounds–so much so that I won't spoil it even here. Mix the above ingredients into a huge, delicious cake, throw a Sea Train Battle Game on top, and dig in.
16. Enies Lobby - 10/10. Maybe some will dispute this rating, but we can all agree that EL is a major turning point for the Straw Hat Pirate Crew. Each character went through a Final Fantasy-style "limit break"–new techniques and abilities displayed when they are most needed. Gears, Ashura, Diablo Wind, Kabuto, "Monster Point," Perfect Clima Tact... the list goes on and on. Perhaps there are a few too many rampages and near-death escapes, and maybe the whole shebang does seem to congeal into an interminable slugfest, and perhaps Rob Lucci's fight with Luffy and the Buster Call scream "FANBOY!!" But Enies Lobby retains a parade of colourful characters and show-stopping action sequences. Taken together with Water 7, it represents the Straw Hats' darkest hour, and their greatest triumph. For those seeking more than mere battling, I point out the recuperation period in Water 7. With the arrival of a single Marine, Oda slaps us with a whopping four startling revelations concerning Luffy's relationships, not to mention an eighth crewmember, bounties all 'round, and a magnificent new ship. Is there a downside, you wonder? Just ask the Usopp fanboys.
I don't have much to say about the new arc, which will apparently take place in the mysterious "Florian Triangle" that stands between Water 7 and Fishman Island. One can only assume that the Straw Hats will uncover/defeat whatever is causing those ships to disappear. I eagerly anticipate Franky's interactions with the rest of his newfound nakama.